jayman,
Lots of good advice here already. I would only add that no matter what you pay, you have a great deal already. The man came to you with a non competetive deal, plus you know the history and the guy who maintained the truck. You can go back to him and ask questions about how he did things and when he did things. Something I know I would put a very high value on.
Like many people have advised. Tell him you want to be fair and you are having a hard time seeing his side of the equation. Throw a price at him and ask how close you are or just ask. A lot of that will be based on your personality and how well you know this guy.
Good luck and we'll let you answer this question the next time it comes up.
Bill Schickling
