Re: Name that boat.
Assume the motors are shot too.
Seriously he should be giving that away, but price wise you should offer him the going scrap rate on the trailer (I know, you want that, just establishing a price point). The rest is just throw in to get it off his yard. He should consider that a favor. There's certainly a lot of negotiating factors there.
If you can pick it up for a hundred or so, part out what you don't want and take a saw to that hull and dump it (or heck, sell the hull/donate it to charity, w/e), you should be ahead of the game.
I'm sorry that doesn't answer your original question, I am no good with Tri Hulls.