5D3 being at $3500, I don't see the 5D2 and 5D drop very much. Especially if Canon decide to stop selling 5D2. But even if Canon continue to sell 5D2, it'll still hover around $1900(my guess) for a new one.
I think drive_75 is right... there are likely to be more out in the market for sale, but the price difference between all three bodies means the depreciation won't happen as quickly.
That's incorrect. The price is plummeting for the 5d2, as it did with the 5dc when the 5d2 was announced. 5d2's are actually selling for $1600-$1750 now, whereas only 2 months ago they were in the $1750-$2000 range. This is nothing like the 1ds3 situation, where prices skyrocketed with the 1dx announcement. The 5d2 is still available for purchase from canon, AND they have reduced the refurbished price. Everybody and their mother is set to sell off their 5d2's when the 5d3's start shipping--just watch the prices fall. We may be in the $1500 range at the beginning of april.
The 5Dc's are another matter. They're unlikely to fall much, if at all in the next year or two. It's already the cheapest FF body available, which is keeping its price high relative to the older 1Ds mk1.