That old one for the original Xbox seemed to never drop in price.
I wanted one for while because I figured I could use it on my mac as well, but it was always around $100.
Well the official one is way over priced and never seems to come down in price but there are alternatives.
That's probably your best bet in fact the original xbox adapter works for the 360 so you could always use it. Plus ive seen a bunch of articles that give alternatives. Like this one from ign :http://gear.ign.com/articles/792/792434p1.html
Or just do a google search for 360 wireless adapter alternatives im sure youll find at least a couple useful articles