I believe Amazon was selling the Rock Band 2 sets for $100 a day or two ago (was not gold box or deal of the day); I'm not sure if they still are, but that is a good deal for the game and the set of instruments.
If you have a local Fry's; some have the RB2 set for sale for $99 right now as well.
The guitar hero world tour bundles should be on sale at Gamestop for $100. So, you can just pick up either set for the same price. I like the Rock Band set more; the guitar seems to be better quality, but they both function pretty much the same.
Edit: This is for the US; I'm not up to date with deals in other countries. Also, the instruments in the RB2 and GH:WT sets are compatiable with the other games.







