I assure that you that short of molecular self-assemblers, these fungus treated violins are not going to be cheap. They will cost well above the average for a new violin, but much, much less than a violin from a classic master.
Demand will probably lower the cost of price due to competition and the related support of research. While demand for cheap violins might be low, the demand for cheap "x made better by fungus" will probably be pretty high, and violins will peripherally benefit.