ONE way is to look at the competition and by testing the market by setting a price. (Supply and Demand). You, as starter, might not be looking for a profit in the very beginning, until you get your foot in the door. Calculating the COSTs, if you want to get into your time, greenhouse maintenance (construction investment), labor .. can be involved. But you can calculate your basic direct costs:
--- seeds
--- starting mix
--- pot/container
--- fertilizer
--- electricity (for grow lights)
--- advertizing (if any)

BY THE MARKET:
Most online mail orders charge about $3.50 per 4 -6" seedling. The same goes for the big box stores in the spring. Sometimes they sell smaller seedlings in 4-pack for about the same. I think the farmers marketers just sell mostly their extras just to cover part of their expenses. They may or may not be making money by selling seedlings. JMO