They are worth a good bit. Some prices are more optimistic than others. eBay has made the dolphin fish pieces more accessable, which I am sure has had an impact on their values.
Salviati and Barovier made glass that is like your lamp. I believe that AVeM made similar glass. Looking at your lamp, I would say probably Barovier & Toso. I don't know if these bases were made with any particular shade. Anything that looks good with them would probably work just fine.