search diymobile's forum... i've seen a few used ones crop up from time to time when i was still fairly active on that forum. it's nothing but audio nuts so there's usually a ton of great quality gear getting posted up.
as far as cost goes... like i said man, it's like buying a second headunit so it's hard. all depends on the type of person you are though and how badly you want it i suppose lol. i mean hell i have no issues spending over a grand on a quality headunit if it does what i want it to do. others out there pitch a fit over a headunit navi set up that costs 600 bucks.