I offer two levels of painting: tabletop and display. I like to keep it simple.
Below are some example pricing. Pricing will vary depending on the specific miniature, and complexity of the painting required.
Tabletop unit of ten miniatures, human sized in 28mm scale: $150
Display unit of ten miniatures, human sized in 28mm scale: $300
Tabletop quality tank, WH40K Rhino: $75
Display quality tank, WH40K Rhino: $150
The pricing will fluctuate, according to how much you would get painted at once. The more minis in an army, that have matching colors, the better pricing will be.