Is a mushroom spawn business profitable and who are the customers?

A spawn business can be profitable because spawn is a recurring need and good reliable spawn suppliers are valued, but profitability rests entirely on your spawn being consistently pure and on building a base of regular customers. Your customers are the mushroom growers in your region, including small home and seasonal growers, larger commercial units, KVK and training program participants, and sometimes kit sellers. Demand grows where mushroom farming is spreading. The risk is reputation, since one batch of contaminated spawn that ruins growers crops can lose you customers quickly, so quality control is also your marketing. Because returns depend on your scale, your contamination rate and your customer base, I would not quote a fixed income. Start by reliably supplying a few local growers, earn a name for clean spawn, and grow from there. Check local demand by talking to existing growers and your KVK, and confirm any licensing needed in your state.