How much should we pay bartender?

We need advice on how much we should pay our bartender. Our venue has been relatively inexpensive for the type of venue and the area, but the add-on bartending Services was going to be $5 a head. Sounds inexpensive until you realize that's $500 for 100 people! For one bartender! So we asked an acquaintance that we are not terribly close with, and he agreed. But when asked how much he would charge, he asked for details first on whether it would be open bar or close bar. As it's open, he says he will make good tips and so he said gas money would be fine. I am NOT paying him just gas money. How much is a reasonable payment? For what it's worth, I'm in the Capital District of New York.