Too many varibles on that one. You'll find a wide range of prices and offers.
Hours on the machine are the biggest factor and if it's a gas or diesel tractor. (Most likely you'll see nothing but diesels.) Farm related equipment has been retaining it's value extremely well the last few years. My father has a medium size JD front wheel assist tractor that is close to 30 years old and he can still get the same price out of it now as what he paid for it in the 90's.
The only real advice I can give you is find units that appear to have been sitting for a little time. See what they have for a price and play hardball to drive down the price. If it's been sitting and they want to unload it, they might take a lower offer.
That or see if you can get any attachments thrown in such as a post hole digger or something you could utilize on your property.