Compiled Results from Other Answers
DnD Next numbers include calculations from both the 1st and 2nd playtests.
Fighter Rogue Wizard Sturdy Wizard
OD&D 11 3 2 -
AD&D 14 6 2 -
3.5 11 6 3 4
4e(MM1) 13 10 7 9
4e(MM3) 11 8 6 7
Next test1 10 8 6 -
Next test2 12 4 3 -
AD&D: improved fighter & rogue survivability
3.5: slightly improved wizard survivability, and pulled fighter survivability down considerably (trend towards narrower range begins)
4e: improved everyone's survivability, though mostly rogue & wizard, further narrowing the spread
4e's MM3: reduced all survivability and tightened the spread again
DnD Next (playtest 1): slight reduction in fighter survivability to tighten the spread even more
DnD Next (playtest 2): major reversal of the reduced spread trend
From 3.5 on, every edition change (including the switch inside 4e from MM1 damage expressions to MM3 damage expressions) has essentially worked to reduce the survivability gap between the toughest and weakest PCs, primarily by bringing the fighter down but in 4e's case by bringing the wizard up. We're down to fighters lasting about twice as long as wizards, rather than the 5-7 times longer from OD&D and AD&D. Rogues have moved from being only marginally more durable than wizards to being about halfway between wizards and fighters. Base wizard survivability has approximately tripled since OD&D/AD&D, and later editions have given them more options for improving it further.
A Note on HP Inflation
As of playtest 1, worries about hit point inflation in D&D Next over 4e appear unfounded: D&D Next PCs last about as long as 4e PCs do when using the new 4e monster damage values, and only slightly less than 4e PCs do when using the original 4e monster damage values.
As of playtest 2, DnD Next hit points are back to pre-3rd standards.
Short answer: No, by RAW
The monsters and NPCs in the monster manual (and equivalent sections in other supplements) do not necessarily follow the same rules as player characters.
This can be seen just by looking at the various human, elf etc NPCs in the monster manual and noting that many of them have differing abilities to those noted for their equivalent player races in the PHB.
Similarly, there are a few monstrous races such as the hobgoblin, goblin and so on presented in Volo's guide, specifically presented as player races. But these do not follow the same exact rules as their monster entries in the Monster Manual (even if they keep to the same basic 'theme').
As to whether it would be unbalancing...that rather depends on level. It is a powerful ability. It is slightly offset by the length of time is required for the summoning, but this generally wouldn't be a big deal because it lasts for an hour and the players could simply summon the elemental prior to entering a dungeon or whatever.
Having a CR 5 creature under your control would be very powerful for a low-level party. For comparison, Conjure Elemental is a 5th level spell (typically accessible by a 9th-level spell-caster).
Ask your DM.
Like many parts of the D&D 5E rules, a lot of specifics and clarifications are left up to the DM to decide. It's very unlikely that you'll get a solid answer from the book about this, because as you point out in your question, even something as simple as "how much does a house cost" is going to be complicated enough to require in-game adjudication by a DM. There are three different approaches to this issue that I can see, all of which are entirely valid and supported by the system.
1. The total cost of the house must cost less than 25,000G.
This approach means that everything involved in the normal purchase of the house has to be considered in the wish. Buying the land, buying the raw materials, shipping the raw materials to the build site, labor, buying appropriate land deeds to make the purchase legal, and anything else that might plausibly come up when building a house using mundane means must be accounted for under the 25,000G price limit. This approach treats wish as a way to effectively give you a 25,000G bonus, as well as speeding up the process of spending it, and gives you both a house and the legal right to own that house.
2. The cost of the raw materials must cost less than 25,000G.
This approach focuses on the wording of wish that says that it creates an object. This wish won't affect the legal standing of the land or provide any other ancillary benefits, but will make a proportionally larger house than option 1. In this approach, the wish is just creating the house itself and placing it where you direct, and thus doesn't need to care about outside cost considerations.
3. It's wish. Do whatever you want.
The wish spell is a big deal. It's a 9th level spell, it has some serious drawbacks, and there's a significant chance that any single wish will be your last. In my games, if a player is using wish on something as mundane as a house, I'm going to let them have as much house as they want, and not bother them with the legal issues. In this approach, the precise GP value of the house or the land that it's on isn't important enough to the game to be calculated precisely, and so is handwaved as "probably less than 25,000G, so let's move on with the game".