If you spoke to the mechanic about bringing your own parts, why didn’t you ask him about labor charges? This is the only fellow on earth who can give you a correct answer to your question.
I suppose you are trying to determine in advance whether his quote might be excessive. What if it is? Insult him and he will never help you out again. He’ll walk away muttering something like “cheapskate”.
My advice is to just deliver the van and the parts, then pay him whatever he asks. You can then decide if you want him to work for you on the next job.