Value of Slant Six Car

I have a friend who owns a 4-door 1975 Plymouth Valiant with a 225. The car is a bit rusty in the quarters. It drives pretty decent, and recently had a new carb put in. I think he said it has around 70K miles on it. He's looking to sell it, but has no idea of the value. I can't imagine it would be worth much, but if you guys have any idea, I would love to hear it.