We don’t currently have prices for lab grown gems, but creating such a price list is an excellent idea.
Your best bet may be to look at the websites of well-known manufacturers of synthetic gems, such as Chatham. They don’t appear to have prices for loose synthetic gems, except diamonds. However, they have jewelry pieces for sale with various lab grown gems, such as emeralds and alexandrite, and list the price for the piece as well as what the price would be if the stone were mined. If you take the (considerable) difference and subtract it from the price of a natural stone with the same qualities, you’ll have an idea of the synthetic gem’s price.