Speaking from experience(as the one providing the service), what tends to go into a general formula is the price for any materials they would need, the time it would take, and the effort it takes. Legally speaking, you are paying for the service and/or the material object if it is something like a talisman. You are not paying for whether or not it actually works. If no good is exchanged, you are paying for a service, and if a good is exchanged, you are paying for that thing. A smart person, to protect themselves from liability, would pretty much say you are paying for the service or good and not whether or not whatever working actually worked, so it really does not matter if magic is done or not.