I'd pay $90.00 for the game and the drum controller. I believe it's been confirmed that you can use a Guitar Hero controller with Rock Band so I'd be set there.
Edit: And yes, it most likely would cost you $300.00 if you wanted all the "official" Rock Band accessories. But if you have Guitar Hero, you have a guitar. You're going to be able to use your headset, so you have your microphone. All you need next is the drum controller.
Edit: And yes, it most likely would cost you $300.00 if you wanted all the "official" Rock Band accessories. But if you have Guitar Hero, you have a guitar. You're going to be able to use your headset, so you have your microphone. All you need next is the drum controller.