That screen doesn't seem very readily available... a quick search suggested the screen assembly will be $500-700 just for the part, meaning that it's probably barely worth replacing, even if you do it yourself (since the whole iMac is probably worth about $1000 used, right, or maybe slightly more?). I would guess you'd get quoted in the neighborhood of $800-850 to replace it.
I dunno... do post a picture of the scratches. It's risky, but the other option at this point might actually be to try something to polish the screen, depending on how badly it's scratched. I guess you could also always try that and then replace it if that doesn't work.