Prices have remained high on this GPU because of supply and demand. They failed a lot. We have repaired four of them just this week after we found out how to do it and got the rework hot air machine to do it with.
PowerBook Medic is selling it for $150: http://www.powerbookmedic.com/parts/661-...
If you want to take the time and learn how to put it in, here is a guide that will work: iMac Intel 24" EMC 2267 Graphics Card Replacement
eBay is offering repair service for $120 here: http://www.ebay.com/itm/APPLE-IMAC-24-20...
If you want me to give your's a shot I'd charge $95, you pay return shipping. No charge if I can't fix it.
I know this is a little SPAMy but I don't do it often ;-) And it is a very viable repair option. And sure beats the prices that Dan's links gave. No offense meant Dan. (After all I emailed you on how to do this repair last week so you could do it!)
Or you can do it yourself with some hours of study, and this equipment:
Board Pre-Heater Hakko FR-870B
Loc-Line Kit Hakko 999-205-02 fits FA-430 Smoke Absorber
Solder Leaded 63/37 Rosin Core
Omnivise Board Holder Hakko C1390C
Arctic Silver Thermal Paste
Arctic Silver ArctiClean