I would look at it an hourly wage type deal. If you were making $6 an hour for 5 hours then that would be $30 a lesson. It sounds fair to me.
I think what I'm going to do is still offer to help with some things at the first barn. They have events that they go to and pass out info about who they are and what they do, and they've said they need people who can go to the events and do that for them. So I think I will tell them I'll do a couple events but I can't come out regularly to help at the barn.
Hopefully my mom won't give me a hard time about this. That's part of the reason I've felt bad about it, because my mom has always told me if you tell someone you'll do something then you've made a commitment to them and you need to do it whether you want to or not.