Concerning stock and prod weights, Ralph Payne Gallwey, quoting an ancient Spanish crossbow manual, says the stock should weigh at least as much as the prod, otherwise recoil will be excessive. As I recall the correct quote is: the stock will recoil against the face, causing wounding and offense.
As for unit weight, most of my light crossbows weigh in at about 6 pounds, whereas the heavier steel ones are closer to 7.5. The heaviest bow I have made was a fat German sporting bow with a @ 350 lb. Digitarc prod. It probaly weight closer to 9 lb, bound-in, but without a stirrup. It was very head-heavy, but the few steel-prod renaissance crossbows I have handled were similarly head-heavy.
When you see ancient bows in museums, look for the grubby area on the lower part of the stock, about halfway between lock and prod. That's the balance point.