Guitar Hero 2 X-plorers can get down to pretty cheap, mine is still my baby for playing Rock Band 2, it's lasted me since GH2 came out and I use it instead of my Strat.
[ame]http://www.amazon.com/Guitar-Hero-2-Controller-Xbox-360/dp/B000NUIYK0/ref=sr_1_9?ie=UTF8&s=videogames&qid=1230654097&sr=8-9[/ame]