If you can still find a store with some of them, some EB/Gamestops were selling used wired guitars for 14.99 a month or two after the PS2 version of GH2 released. However, I had to ask them to check in the back as they had none on display. At the time two local gamestops had none (and said they didn't accept them for trade-in because of size/space reasons) The third gamestop I tried had a single black GH1 guitar, but it was nasty dirty so I passed. However, a local EB had two of the black GH1 guitars and one of the red GH2 guitars.
My understanding is that stores vary wildly when it comes to taking in peripherals and accessories for trade so YMMV.