The easy way

Unless the DM wants to make this more complicated, the Player's Handbook on Spellcasting and Services (PH 129) prices this spellcasting as follows:

• Either a level 9 wizard or a level 9 cleric with access to the domain Travel casts the 5th-level spell teleport [conj] (PH 292-3) for 450 gp (i.e. caster level × 50 gp). Similarly, a level 10 sorcerer casts the spell teleport for 500 gp. Note: Traveling 1,200 miles from the Moonsea to Icewind Dale means at least a level 12 caster is needed to cast teleport at a cost of 600 gp.
• A level 11 druid casts the 6th-level Drd spell transport via plants [conj] (PH 295) for 660 gp (i.e. caster level × 50 gp). Note: An alternative presented due to demographics; see below.
• Either a level 13 wizard or a level 13 cleric with access to the domain Travel casts the 7th-level spell greater teleport [conj] (PH 293) for 910 gp (i.e. caster level × 70 gp). Similarly, a level 14 sorcerer casts the spell greater teleport for 980 gp.
• A level 17 wizard casts the 9th-level Sor/Wiz spell teleportation circle [conj] (PH 293) for 2,530 gp (i.e. caster level × 90 gp and 1,000 gp for the material components). Similarly, a level 18 sorcerer casts teleportation circle for 2,620 gp.

If the goal is to get the PCs from the Moonsea to the adventure site in Icewind Dale, and the how isn't important, the DM should simply assign the trip a gp cost and narrate the outcome. The DM's made the decision that the journey isn't important; instead, what happens at the destination is worth time at the table.

Teleporting a large party on a budget

A party planning this kind of trip probably has at least one Heward's handy haversack (DMG 259) (2,000 gp; 5 lbs.) (and, if not, it should get one), which, because the DMG says it's like a bag of holding, holds creatures for up to 10 min. before they run out of air. (At least, it should—I mean, buy a chicken (PH 112) (2 cp; a few lbs.) and test it (haversack not chicken) first, obviously.) Such a party should also have either a wizard or a sorcerer or a party member who can manage to cast from a scroll a 1st-level spell scribed by a level 1 caster. An arcane scroll of reduce person [trans] (PH 269) (1st-level spell at caster level 1) (DMG 239) (25 gp; 0 lbs.) when used for 1 min. cuts a creature's height in half and divides its weight by 8. Planned carefully, the duration of the spell reduce person and amount of air in the haversack should be sufficient to get everyone into the haversack, teleport, and out of the haversack without fear of bursting the haversack or suffocating.

The hard way

If getting from the Moonsea to Icewind Dale is supposed to be an arduous task, then it won't really be the cost of the spell greater teleport that'll separate the party from their coin, but the preparations the caster must take before making a 1,200 mile journey to a destination with which the caster's unfamiliar.

The first difficulty becomes finding the correct caster. This DM bases this kind of trial¹ on the Dungeon Master's Guide's demographics for Total Characters of Each Class (138). However, the Forgotten Realms Campaign Setting on NPCs in the Community changes some numbers, saying that

Wizards and sorcerers are somewhat more prevalent in Faerûn than they are assumed to be in the Dungeon Master's Guide. On Table 4-43, Highest-Level Locals (PC Classes), treat the sorcerer entry as 1d6 + community modifier (instead of 1d4) and the wizard entry as 1d8 + community modifier (again, instead of 1d4). Just about every thorp or hamlet boasts at least one wielder of the Art, even if that person is a humble hedge wizard.² (297)

Here's how these modified demographics break down:

Highest Level NPC of Each Class  |---------|---------|---------|------------|
---------------------------------| L. Town | S. City | L. City | Metropolis | 
Cleric, Druid, or Sorcerer: 1d6+ |         |         |         |            |
---------------------------------|   +3    | +6 (2×) | +9 (3×) |  +12 (4×)  |
Wizard:                     1d8+ |         |         |         |            |
---------------------------------|---------|---------|---------|------------|

For example, to determine the level of the highest-level NPC cleric, druid, or sorcerer in a small town, the DM rolls 1d6+3, and, for example, to determine the level of highest-level wizards in a metropolis, the DM rolls 1d8+12 four times.³

Thus it'll usually be fairly easy for the PCs to locate a wizard or druid that can cast an appropriate spell in a large city, and it's just about guaranteed that someone can cast an appropriate spell in a metropolis.

The Moonsea's cities and metropolises

For reference, the Moonsea's small city is Hulburg, its large city is Zhentil Keep (Mysteries of the Moonsea 124), and its metropolises include Hillsfar (Mysteries of the Moonsea 47), Melvaunt (Mysteries of the Moonsea 10), Mulmaster (Mysteries of the Moonsea 89), and Thentia (Forgotten Realms Campaign Setting 161). Undoubtedly, there are others, but Forgotten Realms geography isn't my forte. (But even I've wisdom sufficient not to seek spellcasting from Zhentil Keep.)

Once the caster's located, the PCs must convince the caster they aren't luring him into a trap. The caster should charge the PCs for either the price of a discern lies or zone of truth spell or the rent on a rose of kings (Dragon #289 106) (10,000 gp; 0 lbs.).⁴

Assured that the deal isn't some kind of trap, if he's not already familiar with the destination, the caster'll either need to become familiar with the destination or have "a detailed description from someone else or a particularly precise map" (PH 293). This will likely cost more cash. For details, see this question.

Unless the PCs want to try their luck finding a comparable caster near Icewind Dale (Hint: Luskan and Neverwinter are large cities), the party'll have to persuade the caster to stay once he gets the party to the destination. As a DM I'd rule that a truly desperate and down-on-his-luck caster might charge as little per day as 5 gp × his level, while a busy one would charge per day 10 gp × the caster's level × the spell level of the highest level spell he can cast. (In the latter case, such a harried, in-demand caster would argue that he's losing at least that much per day he spends with the party, after all.)

Alternatively, the PCs could arrange to be picked up at a certain destination (including time and day). This would likely entail arranging a contract; for details on doing this, see this question. This will also be expensive, probably costing the price of at least 2-3 more greater teleport spells.

Like Dan B mentions in this answer, it's also possible for a caster to charge the party for his own return trip. This DM would hesitate to do so because a caster willing to zap an adventuring party somewhere should be competent and confident enough to zap his own way home, but if the DM is looking to truly squeeze the PCs out of their cash, that's an option, too.

And, of course, a smart caster takes all this money from adventurers in advance. They're adventurers; they have the lifespans of mayflies.

Anyway, right before departure, a caster may or may not explain that sometimes even the spell greater teleport for no reason goes nuts (that is, technically, greater teleport still requires a roll on the teleport spell chart as a false destination or mishap can still occur even though there's no chance of being off-target), and sometimes teleportation magic is blocked for reasons unknown (like because "[a]reas of strong physical or magical energy may make teleportation more hazardous or even impossible" (PH 293), i.e. DM fiat).⁵

If such weirdness does occur, the caster'll explain it's not his fault and another attempt can be made... for the same price as this attempt, obviously. Maybe tomorrow. If he's not busy. In the meantime, would the PCs be interested in this scroll of greater teleport?

As previously mentioned, there's no reason for the DM to make travel difficult and expensive if the DM wants the PCs at the destination quickly. These are just the kinds of arrangements the DM can impose if he doesn't want the PCs relying on outside aid for instantaneous long-distance travel.

Alternatives and splatbooks

There are dozens of different methods of instantaneous transportation, from the did-they-really-print-that-and-what-does-it-do-exactly? 7th-level Drd spell master earth [trans] (SpC 139) to the I-can-take-1-person-per-2-caster-levels-without-stuffing-you-in-a-handy-haversack 6th-level Sor/Wiz spell stormwalk [conj] (Stormwrack 122) to borrowing (perhaps creatively) someone's mirror of mental prowess (DMG 262) (175,000 gp; 40 lbs.). One answer can't cover them all. However, the examples above are likely the most familiar and straightforward.

Also, there are portals

Wizards of the Coast published the Perilous Gateways Web column on known portals in the Forgotten Realms, including a four-part series on portals of the Moonsea. None detailed lead to the Sword Coast, but they're useful for inspiration.

¹ Some would call it an adventure!
² Staggeringly, that means wizards are as common as fighters and rogues in the Realms. Considering even the bottom-of-his-class Int 11 level 1 wizard has a treasure valued at 600 gp somewhere, there's a lot of money to be made mugging low-level wizards on the streets of the Realms. (I kid! That's a really bad idea.)
³ In most campaign settings, the highest level wizard is a Wiz16 (or four) in a metropolis, putting 9th-level spells beyond the reach of PCs except through scrolls, the typical 9th-level spell scroll costing 3,825 gp and available in towns the size of small cities and larger. No, I don't know how that happens, either. Relics of a bygone age of mighty wizards, maybe?
⁴ The item's the only absolutely surefire yes-you-really-do-have-to-tell-the-truth! method I'm aware of.
⁵ A druid using transport via plants is in both a better and worse position: distance is not a factor nor is familiarity, but the local and destination flora is. I am even less of a Realms botanist than I am a geographer. Consult, instead, your local druid circle.