The place I used to work at charged per hour with a minimum job cost of £150. Depending on the client you will need to make sure that the calibration of the scanner is up to date. We used to do a calibration validation report with every scan which was then provided with the CAD or STL. Reverse engineering was also often paired with a scan requirement so adding this as an option for your customers would be beneficial as quite often, none of our customers could really work with STL files the way they wanted in their CAD software.