I'd charge about $10 per hour, depending on the size of the tank. For a 10-55, I probably would only charge like $5 per hour (since I'm so used to doing wc's completely manually on my 220g, maintenance on a tank under 55 gallons would be nothing to me), but for like 75+ I'd say around $10 per hour. For 200+ it'd start getting a lot more expensive.
you would have to calculate gas and a lot of other stuff. Make sure you don't cut yourself short, people pay a lot for fish tank cleanings. I would call an area around that does cleanings, and as them for a quote, then go a little cheaper.
I know a guy that charged about $50 a hour for med. (55-75) and had a lot of costumers. But he basically ran the tank and all they had to do was feed the fish. But that was saltwater so I am not sure how much he would have charged for FW.
$50-$75 hrs or $1-$1.50 per gallon depends on tank size, but never charge less than $20 regarless how small the tank. Your job is skill, not general label.