We're seeing 5D2's go for $600~750 often these days now. It won't be much longer before they dip down a bit more.
And by then, the 5Dc will be hard to find in general probably (like how the original 1Ds is now).
I think "full frame" still carries a lot of weight with people shopping for these cameras. They're hyped up on the web and in media in general. Yet, more and more crops are being produced. For this, the 5D2 still maintains a little extra value compared to a similar crop. If you took the full frame sensor from the 5D2, no one would even pay attention to it for any reason because its features are not competitive today at these used prices. If the 5D2 was an APS-C or APS-H, I bet you'd see it at less than $400. At that point, a 60D is likely a stronger camera in general from an AF perspective alone. So again, full frame still carries a lot of weight when it comes to cost. The 5D2 and 6D1 will continue to be less than $900, but probably not dip below $600 for a little while more. By then the 5D3 may become a $1k used camera.
I think this pattern continues with the 1D series. All the old 1D series that were APS-H are dirt cheap. They're still great cameras, but the menus on some of them are archaic and not fun and have big heavy takes-forever-to-charge batteries. So those are dirt cheap even though they still are commonly bought/sold on the used market. Then the 1D3 comes along with a modern menu system and modern batteries, yet, it's a cheap pro camera. $500 or less easily these days. And it's a beast of a camera for what its used price is now. But, it's not full frame. The 1DIV is under $1k these days, following suit, also as a crop. Then you see the 1DX, a full frame, clinging to that greater than $2k+ mark, even with an end-of-life-shutter and it's quite old in terms of its release. I think if the 1DX were an APS-H, you'd see it's used price much, much lower, especially with the 5D4 around.
The old 1DS series that are full frames, carry a high used price tag compared to the APS-H version. Significantly higher. 1DS costs as much as a 5Dc often and is hard to find. 1DS II carries itself still anywhere from $400~600 and is not easy to find. And the 1DS III is crazy expensive considering a used 5D2 is cheaper and has a lot of advantages over it (other than AF and build). For the used price of a 1DS III you can get an used 5D3. Yet, the 1D3 is dirt cheap compared to the 1DS3 and the only difference is full frame vs APS-H. Full frame just helps hold value for some reason.
Very best,