Fair Fee for Trailering?

I recently went to my first show. I went with 3 other riders in our trainer's trailer. The show was 30 miles away. I figured we'd all be paying for gas and a little extra for wear and tear, but was stupified when I received bill for $100! I'm sure she only spent about $80 max for gas and made $400 from the 4 of us. She charged us each $1.50/mile. Is this normal practice, or is my trainer trying to make a serious profit here?

Most commercial haulers charge $0.50 to $1.50 per loaded mile.