Are you sure it's a 12 hp? That's pretty low power for a 42" Z. This is a new Toro 42" with a 19hp Kohler:

It sells for $2500
HERE.
Considering it's 4 years old, just 12 hp, if this is like yours, I honestly don't think you can get any more than you still owe on it used, not with new ones selling for $2500.
I have a Land Pride 48" with a 20 hp Honda that's a similar design (only quite a bit heavier) that cost me $4800 new and I'm not sure it would bring $3,000 used, and it's still under warranty.
John Stricker
[This message has been edited by jstricker (edited 06-04-2010).]