You can definitely pick up a non-Microsoft branded one (any plug-n-play gaming adapter works) for about $25 off of ebay or Amazon, or about $40 from Fry's. I bought a D-Link Gamerfuel wireless adapter, and it works just fine.
I bought a cheap generic one for our upstairs 360 and it was a pain in the ass to setup. Ended up returning it and getting a used Microsoft one from Gamestop. Worked perfect, had it up and running in less than a minute.