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Well...can you buy the game with just the drums? Or maybe the drums and mic? Or do you have to get the whole shabang?
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AFAIK, you can't buy the game with just the drums. It's only available as a kit (with guitar, mic, and drums) or with the game only (which you would use with the controller or with the GH2/GH3 guitar).
The drums will be available separately, but not until next year:
http://www.amazon.com/Rock-Band-Xbox...dp/B000TT2D2A/