Well the rumors are around $200 for the whole kit, but those are just rumors. However, I'm almost positive that they'll also be selling it in bundles with only a single peripheral, such as just a guitar or just the drum set, for prices similar to that of past guitar hero games. Then, if your friends want to play, they can bring their own controller, or whatever other arrangement you wanna work out. Honestly, if they didn't allow this option they would be extremely dumb, as they have stated that they want to make it as affordable as possible.
I'm more excited for rock band because it looks like it'll have just as much single player content as guitar hero 3 (minus the boss battles, but I really don't care about those), in addition to multiplayer goodness. In fact, counting the insane support they're gonna be getting for downloadable content, plus the option of buying a different peripheral to play the game through again in a different way, it might even have more.