If it comes with T-Case, then it's worth about $550 from a junkyard (around here), or$350 from a private owner. With out T-Case, I wouldn't pay anything more than $250, and probably only $200 becuase of unknown condition. Give the guy a "woe is me" story, and see if he'll go to $150.The last W56 I got, I spent $60 on. This was a rare find, but they are out there.
If you could get someone to give you a loan, you could offer the guy $400 for the truck, and sell off the other parts from it to make your money back.