Part of the reason right now is because the Christmas season has inflated the prices to higher than normal. The other reason is that SoTM is now out of print and Heroscape in general is more or less extinct. Even though the second master set is easier to find than the first, it still has gone up in value according to how hard it is to find.
It's more than SotM that's out of print - the entire line was canceled last year by WotC. What's out there now is all that there will ever be. You're seeing the effects of dwindling supply.