Okay, I’ve got an econ background– let’s do some nearly research free speculation.
Klingon. A quarter of a million copies of the Klingon dictionary translates into $3,250,000 revenue, split up between Paramount Pictures, Simon & Schuster, Marc Okrand and book stores. There was also a boxed software application for teaching Klingon. No idea how well that did, although I vaguely remember some harsh words for the quality of the software. There has also been two audio tapes, two KLI translations (Gilgamesh and Shakespear) Altogether, I’ll bet there was $5 million in revenues generated from the language. That said, I doubt any individual made more than a few thousand from the project.
Esperanto. This is very hard to put a dollar value on. It’s not making Zamenhof any money now. Presumably lots of people still make some small change publishing books in and about Esperanto, I’ve seen them at the library and own one or two. Since Esperanto is a somewhat established language, it is like saying how much money is being made speaking Dutch. Should we count the GDP of all the speakers? The revenue of the publication industry? Maybe only the revenue of educational materials and teaching?
Elvish, Huttese, Etc. $0. No one would pay a cent for these if it wasn’t embedded into a fantastic book or movie. Ok, if we want to be charitable, it would be worth the difference between the revenue the books and movie would have made with and without developed languages. In all honesty, they could have just used reasonable sounding random noises. I’m sure many if not most LOTR fans don’t realize that the languages portrayed are described as completely as they are.
Dead Languages. Same economic and social situation as conlangs, except no inventor. That is, dead language are language spoken by no one, but for some reason–completely outside of the time, place and social context of the original speakers of the language, people decide to start speaking it. Latin, Pali, Sanskrit, Hebrew. Mostly studied as a byproduct of people trying to read the source document of their favorite religions. Most money made by teachers & seller of educational materials. One key feature of successful revivals of dead languages is the need to get sparse and widely distributed people to communicate with each other. Since they already are of the same group (by religion), they don’t need to worry if the language is easy for it’s adoptees (concerns of the design of Esperanto)
Conlang Revenue Sources
Languages for hire. Tho Fan earned the linguist who wrote it $2000– but for 4 months of work! Talk about underpaid.
Conventions. Probably the most & easiest money that could be made out of suddenly popular language is through conventions. I’d pay a convention fee and travel costs to go to a toki pona convention–if real life would let me get away with it.
Collectibles. $50 and $250 for a copy of the Lojban book!