A lot depends on whether your state regards it as a fixture or real property once the axles are off it, which could be different in different states. Go back to them with a counteroffer of what you think it's worth minus the cost of hauling it out and see what they say, with 'Splitting the difference' being a perfectly reasonable end point after the ensuing haggling is done. Worst case you can take their offer, but you might as well make them work for it and you might well come out better on the deal if you do.