A lot of it really depends on what you paid for the unit when it was new. From list prices, it seems fairly common for a new tractor to be discounted about 10 or 15% from list when it's originally sold. Depreciation on these units seems to have more to do with what the best possible deal on a comparable new one would be, and what shape the used unit is in.
I've seen plenty of people get a fantastic deal on a new unit and then a few years later sell it for more than they originally paid. I've also seen a couple of people pay list for a new unit and lose more than 20% when selling a year later.