From what I can tell the small Gilson Opals dont seem to be that expensive, based on a few links I saw glassblowers exchanging. Unless im looking at the wrong type of Opals, but they said Gilson and were for coe 33 boro. About the big fee glassblowers put on adding opals, I'd say it just depends how much the glassblower thinks the option is worth to them. One could say not much and another could say a lot. Not sure if it's easy or hard for a glassblower to encase one. I've seen some diamond and cubic zirconia encasements, pretty baller if you ask me!