grandmother watching baby

This Friday my husband and I have a concert and I asked my grandmother to watch my daughter (3 months old) this is the second time I ask her and she comes over to my house to watch her. She lives about 15 minutes away so she isn't that far. I felt bad the first time since I never like asking anyone for favors and tried giving her at least $20 for her gas. She didn't take it. This time something came up and she's going to be in Rialto which is about an hour away. She said since she promised me she would watch my daughter she would drive back to LA that day and leave the next morning. So instead of canceling she figured out a way. I feel so bad that she's driving so far just for one night. So how much money should I give her this time? I want to add that I know her very well and she will be accepting the money this time because it's the second time I ask her.