You should create a contract based on how many max hours you think you could work for them in a month. And then base it on that dollar amount and give them unlimted hours of service/month.

ie:
You believe that the most hours you would work for them in a month was 10. So 1hr @ $40 = 9hrs @ $30 = $310. Then you could give them a monthly contract of like $300/mo unlimited service.