mother-in-law: need some advice!

Lexie
mother-in-law: need some advice! 
Recently my better half's parents went on a week vacation. I offered to help out with the three younger kids driving them to school, practice, and to keep the house up while they were gone.(take care of the cats, dogs, plants, do chores, etc..) after they returned his mom insisted I take gas money for driving them around during the week. That seemed fair,  about $80 was spent on gas. She ended up giving me $300. 
Need some opinions! Do you think it would offend her by attempted to give it back? I don't feel comfortable having it. 
I told her it wasn't a bother and I enjoy spending time with the kids and I did not want paid but she really insisted. 
When I took the money I was under the impression it was only about what I spent for gas. 
Anyone out there a daughter-in-law or  mother-in-law yourself that can offer me any advice?