You know, it's possible to pick up a used HD 7970 for about $100 if you spend a few minutes looking. If price is a concern, that's going to be competitive with all but the very top-end GPUs you can buy new.
Especially if this is an existing machine that you're looking to upgrade ... you didn't say, but since you are only looking to buy a video card, I was assuming this is not for a completely new build.