Well the best way imo would be this:
Calculate the time each model takes YOU including:
-Cleaning/Priming/Basing/Painting/Packaging/Going to the post office
Calculate the materials you use:
-for example a brush may last for 25 models
-a bottle of paint 15 etc
-grass tufts 30 models
-packaging for 5 models so on..
Set your hourly wage.
Then (time per mini x hurly wage) + cost of consumables per miniature = Your price/per mini.
It's kinda suprising on how much it adds up per model, especially if you are doing a decent paintjob. If you can't find people of your price range (or you can't speed up your painting) you'll end up working for way below minimum wage and probably giving it up after a while and/or hating painting.
All said, best of luck and many happy clients