This has been out of stock for a few weeks and they are currently selling for $50-80 on eBay:
http://www.amazon.com/Xbox-360-Rock-Band-Midi-PRO-Adapter/dp/B00409SOD2/ref=zg_bs_videogames_41
Edit: Looks like, shortly after posting this, they're out of stock again. Sorry
http://www.amazon.com/Xbox-360-Rock-Band-Midi-PRO-Adapter/dp/B00409SOD2/ref=zg_bs_videogames_41
Edit: Looks like, shortly after posting this, they're out of stock again. Sorry