I think this is always the challenge ESPECIALLY when you sell a service.
I think it's important to shop around and see what your competitors charge in your area. Determine what they provide for their service and use it as a guideline for your's.
Then, survey your customers. After you complete a transaction, ask a few questions of them (this can be an email survey or online if you like -or just casual discussion over the phone). If you provide B2B services, then your clients will understand the need for feedback and may copy your idea!
Ask them: did you feel you got good value for this price? Have you purchased this service before - what price range did you pay? would you pay more? What would make you come back?
Tell me about what you do for living, maybe I can suggest something more specific