Does anyone use the Credit membership type and if you do, how do you price each service?

i.e. in ...admin/service_list.php

Let's say your monthly membership is $20 or £10, what percentage of that do you charge for the various services and what do you base your prices on?

Seems to me I have a rising number of Credit members and I'm wondering if I'm either giving good value for money, or under charging for certain services