Check out Amazon for some of the premium instrument add ons. I just got the Mustang guitar for under $70 brand new, and the keyboard is available for only $28. I used to play all the time, then kind of set it aside when I decided that the pro mode wasn't worth investing in. Rockband Blitz got me interested again, and the cross functionality between the games songs is pretty cool, and the downfall of the music genre has dropped the peripherals down in price by 50% or more. I'm planning on using it to get my kids interested in music and instruments, hopefully a springboard into some music education for them.
I've been playing Rocksmith a lot and if it wasn't for that I probably would like Pro Guitar with the Mustang..but honestly after using a real guitar it feels like a step backwards trying to get used to all those buttons.