I free lance as a drummer, and work with in scenarios like that all the time.
There are tons of different factors: location, timing, do you have to learn songs, do you have to bring your own kit, is there a rehearsal, et cetera.
I'm in Boston, and typically, a musician would charge $100 for the first 2 hours, and then maybe $50 or $100 an hour after that... so this gig would be about $100. If there's a rehearsal, I usually charge $20-$50/hour of rehearsal, assuming it's close to me.
There's going to be a market rate in your area though, and you don't want to overcharge, especially if there is the potential for more gigs.
That being said, typically the client will have the pay worked out up front. I'll usually get a text, email, FB Message, whatever, that says
"Hey, can you play with us Nov 10th 6pm-8pm at this Italian Restaurant? $100 + dinner and free parking. Mostly standards, we'll bring charts for some original tunes"
Usually the groups knows how much they can get paid when they book the show.