I think it'd be best to pay something towards it because if the situation isn't working as planned, ie the neighbor is often late, calls in sick often, etc. you have a little more leverage. Whereas, if she's doing it free, beggars can't be choosers & all that. Agree with oldtoyota that basing it on the federal reimbursement rate seems ideal, that way if your neighbor drives a Hummer you're not subsidizing it.