It depends on circumstances. The idiot's genius's at Apple quotes me over $1000 to fix a fan (due to a small dent in my case, which led them to want to replace my HDD and CD drive as well). $50 later I swapped my bottom case, and Applecare took it upon themselves to replace my CD drive, fan, screen, and power button, for free. Interestingly enough, once there wasn't a dent on my case, their tests stopped detecting my HDD as ready to fail.
In short: Apple over charges for repairs. I could replace a fan for $6, or have Apple charge me $1000.
In general though, Apple's markup for repairing stuff is outrageous. I think they charge somewhere around $200 to replace a hard drive out of warranty, but if you do it yourself it's more like $50. So you feel like Applecare pays for itself after a single repair... but if you can get the parts off of ebay and do it yourself, it's only really worth it if your screen or motherboard go.