In addition to the pervious post you could also go to a shop that sells games such as gamestop which might have a game disc repair machine. i got my copy of BO repaired for about $3.00. The game isn't in the XBL marketplace yet, but it should be added about halfway into the games lifecycle.
yep, you can even buy cheap manual buffers for about a £1. They work by polishing of the top layer of plastic so you have to be carefull when using the shop machine one's because your disks get thinner and thinner