I think in 2/3 years you're going to see a lot of E9x M3's on the market. Mainly because of couple of factors, current leases ending and a lot of existing owners trading up for new M3/M4's.
I think the sweet spot is the time between end of production of current E92's (likely July 2013) & prior to the next gen M3/M4 hitting dealer showrooms. Once the next-gen M's hit the market you're going to see a lot of existing owners trading up which will mean a large supply of E9x's on used car lots.
It all comes down to supply & demand.