I’m definitely not an expert on the economy but I’d imagine you’d want gems that benefit greatly from increased level and are meta. Arc would probably be my first choice. Elemental Hit I haven’t looked at but I imagine it does well with level 21. Then any popular support; added cold maybe, gotta have them shatters.
I’m irritated at my rng as I keep getting the doryani’s institute but never have a level 20 gem to put in it much less a 20/20. I’m not willing to spend the money on a 20/20 to take a chance on hitting it big. Meanwhile I’ve only gotten the corruption chamber thing once and I’d love to be throwing things on there.