There are different ways to charge for bookkeeping services. Most will charge by the hour, which if you are paid as a employee is the normal way to charge. This is fine as a employee, but very old fashioned for a freelance professional bookkeeper.
Bookkeepers can be experienced, and many have also had formal training to improve their skills. For these reasons a freelance or self-employed bookkeeper should never charge by the hour. Instead charges should be done per transaction in varying formats. It is the fairest way, as a good bookkeeper can carry out more transactions than a poor bookkeeper.
If you are a bookkeeper or accountant who does wants to quote a fair rate, a good starting point would be to get this pricing calculator software from “More Than Bookkeeping”. A simple one off charge, and no monthly payment to find.