The system is designed to accomodate this
...but without the DMG it's a little tricky.
The science:
Basically, each enemy has an XP value. This is how much XP it's worth when it's defeated (divided among those who defeat it), but it's also useful for building encounters.
Here's how you build an encounter in 4e: You take the XP value of a "standard"-type enemy of the same level as the party, and multiply that number by the number of PCs in the party. The result gives you a "budget" that you use to "buy" enemies to create an encounter of average difficulty (the party is unlikely to die, but will expend a noticeable amount of resources --consumables, healing surges, daily powers-- during the fight).
For a more difficult fight, increase the level of the standard-type enemy whose XP you're using as the baseline multiplier to get your budget, up to four or five levels above the party. For an easier fight, drop the level down by three or four. The extreme ends of this will produce boss-level fights, or make-the-players-feel-invincible routs.
The art:
The DMG recommends actually using enemies up to five levels higher than the party for boss fights, but in my experience this is more frustrating than interesting; it's better to use "solo" type monsters of the party's level. The challenge level will be similar but more fun.
Combine standards, elites, and minions for interesting fights. Minions die quicker and elites last longer, so if there's an NPC or ability you want to be present throughout the fight make it a tougher monster type.
Use soldier (defender) and lurker types for longer more drawn-out battles, brutes and strikers for shorter, more intense fights.
If you've got a combination of enemies whose abilities support each other in significant ways, or you're adding strange terrain, remember that this may make the fight harder than its XP budget will imply.
The mechanics of monsters changed partway through 4e's tenure
With the publication of the Monster Manual 3, monsters got their hp reduced, their damage increased, and their powers were made a bit more interesting. This makes fights take a little less time while being a little more tense, but the overall resource drain per fight is pretty much the same. If you can get your hands on them, use post-MM3 monsters whenever possible until you're familiar enough with them to adjust the earlier monsters to fit that ethos. If you can't, don't worry about it too much. You'll learn to fiddle with monsters based on experience, and until then the fights will be a little more tedious than they'd be with MM3 monsters.
To that end, seriously consider a D&D Insider subscription. It provides a searchable compendium of every mechanic --rule, monster, item, class, race, etc-- ever officially published, a solid character builder AND a solid monster builder, and downloadable access to all the Dungeon and Dragon magazines for 4e. AND all the errata are kept up-to-date across the compendium and builders. I was suspicious of the service at first, but quickly found it to be nearly indispensable.
3k at level 8 is abysmally poor.
I can see why your PCs are clamouring for wealth, it's not vanity, Pathfinder is balanced around wealth being used for magic items for survival so they are going to be struggling to survive.
If they are point-buy-20 or higher they should have over 66'000gp by this point.
Things you are going to have to do to reduce the difficulty:
Limit high crit weapons
As you play the game longer and longer the chance of any player ever getting hit by a crit goes up to something like 80% (this is ever being hit, not any single chance). Also, critical damage can go up hugely as well. As you have been so stingy they cannot afford anything like a Buffering Cap you must never have weapons with 3x crit range and ideally no 19-20/2x crit range either. They aren't going to be able to take a critical hit and you have left them no opportunity to protect themselves.
Limit weapon damage
Because you haven't left wealth for things like Shawl of Life Keeping to give a buffer for general damage loss you need to dial things back a lot. If game guides would suggest enemies armed with greatsword (2d6), arm them with something like a Longsword (1d8) instead.
Makes things close yet environment to your PC's advantage
The utter lack of wealth of course is going to limit PCs ability to extend the range of their attacks, for example Rogue could never afford Assassin's Sight, so you're going to have to keep things a lot closer yet leave plenty of leeway for your players to disengage or use a height or cover advantage. They are going to need it.
Nerf enemies' stats
Your players should have been able to afford items to boost their core stats by now but obviously cannot. So knock a few points off enemy NPC strength and constitution. Or at least impose some limit like they've been fighting and are lower HP already or are suffering from a disease, tired from travelling, their weapons are damaged or are inherently intimidated by the PC's combat prowess (see sickened, fatigued, broken and shaken respectively).
Still, all is not lost, there's still a lot of very good cheap stuff out there that I can recommend that will be appreciated despite their low cost.
This very cheap item can be hugely appreciated should they be in fear of falling.
Ammunition can be enchanted in groups of 50, so individual units of ammunition would cost 1/50th of enchanting the lot. This can be a great way of cheaply handing out single use bonuses such as Bane Bolt/Arrow against a particular troubling enemy type.
Another cheap item of huge use IF you are a generous GM and give opportunity to use it such as flat roofs nearby that with contracting rope they could quickly get up onto.
Short of being able to boost their Constitution or have a magic item buffer their HP, this will have to do for the big fights. This allows them to stay conscious but staggered when they go to negative HP and Troll Oil stops the steady tick-tick loss of HP when reduced to sub-zero HP.
Very cheap item but gives a considerable buff to protect them from being grappled.
A cheap poison with a very useful effect in how since it requires two saves to end the effect it always works. If your PCs are worries about self poisoning then Blackfinger Paste can cover that concern.
Low CL Potions (50gp+)
There are many 1st level spells which are really good but hard to get out and are viable even at the absolute minimum caster level.
- Touch Of The Sea: can make up for inability to buff stats to be a good swimmer.
- Enlarge Person: is a 1 Round to cast but as little as a Standard action to imbibe
- Blend With Surroundings: great for your rogue when he gets into a pinch.
This is a really good way to give your players the edge in the long run, especially if you are unwilling to drop high value items.
Be generous with traits
This could be a houserule or just a favourable interpretation of the rules but one thing I do is allow players to get a new trait every level and ignore the limit on how they cannot have more than one trait per "list". Considering there are over 1000 traits and normally players can only pick two of them at the beginning of the game that makes this a rather untapped resource.
It can reflect how the PCs have been developing as characters in acquiring new traits. This can do a huge amount to help keep them viable despite such a paucity of wealth.
Best Answer
Your question asks how to "determine CR", but it sounds like your actual goal is to figure out how much XP to award.
Rules As Written, you only award experience points for "defeating monsters, overcoming challenges, and completing adventures". Unless the players defeat some of the enemies involved in the challenge, Rules As Written there's no XP reward.
However, you have the ability to give your players Story Awards:
If you decide to not do the Story Award for the battle itself (because you're worried the players might not try very hard), your other option is to add some sort of achievable goal to the battle. The players can't avoid capture, but they can wreck the villain's favorite magic item, or they can thwart the villain's plot, or they can kill some of the villain's henchmen. (It's a good idea to add a goal like this anyway, so that the players don't feel too frustrated by having to fight a losing battle.)
If it's too difficult to cue the players in to what is expected of them, consider simply pausing the action and explaining what's going on out-of-character: "this is a difficult fight and you're probably going to all get captured. But if you can find a way to thwart the villain, you can still get experience points for that, and it could help your group out in the future."