lotsa different BM protos around, imho most are worth about what a std model would be, especially if the are similar to or the same as the std model, like the 806 AFCK11 protos, exactly the same as the production stuff and worth imho exactly the same price, on the other hand some are quite different (ie the 710 protos with aluminum ano'd handles, different thumbstuds) and they are worth, imho, a small premium over the std ones. imho there are a dozen prototypes that are exactly the same as the production ones for every one thats like the 710 and is different FWIW, i know the 710 one is the only one i have seen which varied a lot from the std one.
so, imho, some are collector items, some are users, prototypes are just that, prototypes to gauge interest and practicality, and are the 1st run before std production runs, usually, then ya have pre-productions, 1st production runs, etc, some of those are serial numbered/some arent, they too usually are worth no more than a small premium, maybe. prototypes are usually not serial numbered but are instead dated ie "june 1998" or the like. as far as numbers made i have heard that BM makes in the vicinity of 300 or so, and ships/sells them to dealers and sales reps/magazine writers and the like.
prototypes, pre-pro's, and 1st pro runs are usually only worth something to collectors.
i used to collect prototypes and at one time had a dozen or so, still have a AFCK & a 710 but have swapped all the others off thru the yrs.
so, imho, the great majority have value only as users.