It"s like farmallboy15 says, it"s all in the worth of the moldboards. Check them closely for wear and broken parts. If the coulters are all there and the landsides not too worn, then the rest of it can be found or fixed. The lifting hub is easy to take apart when you get it home, and simple to repair.
With all that, then $100 for starting offer, maybe up to $250, $300...depending on how bad you want it. Four bottom trip type plows are not as common as 2-3 bottoms. Wish I had a chance at it.
Tell him you will fix it up real nice and then when you plow with it, you will invite him to plow with it a few rounds. Keeping it from the salvage guys is an added goal.
LA in WI