Just to expand on what Sam said;
You need to take into consideration all the costs you may incur in the *near* future when you buy the car, (can't expect them to drop the price because it might need an engine rebuild in 150,000 k's, but if it will need an engine rebuilt in 30,000 k's you could negotiate;))
Also, if they won't budge on their price AT ALL, you need to weigh up for yourself whether it is worth it with all the hidden costs, or whether to keep looking
Also things like;
General Condition - If it is due for its Major Service at 100k which includes, timing belt, water pump, cam seals spark plugs etc & will cost at least $1500 ... now while you can't expect them to take $1500+ off the price of the car because of this (it is a cost you must bear when you buy it, and its part of the reason why it isn't overpriced to begin with) you might be able to use it as leverage for negotiation.
If there is a loud tappet noise in the engine, this can be fixed up while doing the major 100k service for a little bit extra cost while they have the engine in peices, otherwise it will be fairly expensive
Suspension Components - The springs / shocks / bushes / droplinks might be in need of replacement in the near future & can cost $1500+
If they're in desperate need of replacement & the owner won't budge on price & the price of the car is not low enough to justify having to spend more money to fix it up, then just find a different car which has had the suspension replaced / fixed or where it is in good condition
Interior & Exterior - If the interior & Exterior are in poor condition, either don't buy it (because it will cost a few $$$ to fix it up) or, get them to lower their price pretty drastically if it is too high.
Things like worn / stained seat covers, rusty, chipped, dented exterior panels, interior panels & the biggest one of all - cracked stereo surround
($300 new) Budget $500 for new leatherette seat covers, $3000 for a new paint job & panel work to fix dents.
General wear & tear on mechanical parts that *might* happen in the future won't affect the price, it is just an expense of owning a car & unless the mechanical faults are obvious, or are found at or before the time of purchase you can't expect them to budge on their price
As Sam said, some cars are underpriced at 10k, some are overpriced at 15k, some are even underpriced at 15-20k. It all depends on; How unrealistic or desperatethe seller is, & what someone will actually pay for it.
Mmm sounds like a good idea then.