A thought for you. I'd say charge on figures based on a three tiered schema.
Tier 1 (Tabletop Mass) - $2 per fig. Figs will be basecoated with primer, then the predominate color. 3 - 4 additional colors and one wash coat will be used to paint all major details. One coat of clear matte applied. Custumer to apply additional coats if desired. $20 to get a full squad table ready with minimum of fuss and muss. You should be able to do 3 - 5 figs per hour, assemblyline style.
Tier 2 - $4 per fig (Elite Units). Figs will be primed, then individual colors applied. 4 - 6 colors on average. Possible multiple washes and drybrushing included. One clearcoat applied. These units will be the flashy ones on the table. $40 for a really nice looking squad. You can probably do 2 per hour at this tier.
Tier 3 - Commission artpiece (Individuals and special figures) - $8 per fig. Fig will be primed, lovingly painted with all the heart and soul the painter knows how to give. Two gloss coats and one matte coat applied. Dr. Diana or Valerie Duval never looked so sexxy. Dammmmm Sexxy! Painting time is going to be dependent on the fig, and how far you want to go with detail.
Treat rider and mount on cav figs as seperate figures. Add $5 to Tier price for small vehicles. Flocking available for an additional $1 per fig. Extra clearcoat above Tier pricing, add .50 per coat.
...and charge Philly 1 MEEEEEEELLLLLLLLLIIIIIIIIOOOOOOONNNNNNNNN dollars for his ships! (Or, $4 apiece for basecoat, wash, drybrush and then some detailed color applied strategically.) I have not painted spaceships before, but based on a few games of Silent Death, I think you can crank out two ships in about an hour, and they will look 10 times better than bare pewter.
Feel free to adjust up or down depending on your actual time on target, and what you feel comfortable with. Long and short of it, you should be fairly compensated for skilled work. Don't be afraid to ask for fees that will allow you to make a decent wage for your labor.
Good luck with it, and I wish you best success.