The tractor is a Dynamark according to the logo on the grill. As far as value, I do not know enough about them, depends on the condition.
As someone who has done a large amount of Urban Exploring both in the USA and Germany, Iraq, and Korea, I would be very cautious about approching the property owner about this tractor. Most property owners who have gone to the trouble to install trip wires have already suffered break ins. While you and your friends may have the best of intentions, to most law enforcement officers, there is little to no difference between urban exploring and breaking and entering. Just be very careful. I personally would not take the risk for a dynamark.