My XBOX is sick.

[Nu-B9979]

I'm having trouble with my xbox. about 30minutes to an hour into most games is get a message saying "Disc may be dirty or damaged." this has happened on two brand new copies of ESPN football 2K4 and on a brand new copy od Crimson Skies. I called XBOX and they want me to pay $91.50 to fix it because my 90 day warranty expired at the end of January. Does anyone else have a similar problem? Does anybody know the best, and cheapest, way to fix it?