Do you mean an interactive sport game like you’d find on the Wii, or do you mean something like an EA sport game?
Either way, look for used copies first. Check on Craigslist or your local classifieds site to find a regular joe selling off his copies to score one real cheap. I highly suggest NOT looking at a video game store’s used copies, as they often sell their used games for 75% of the regular retail price, and you have to shell out an extra few bucks for the guarantee so that when you take it home and it’s broken and doesn’t work, you’re not totally screwed.