I had this same discussion with the owner of my LGS a few weeks ago. This is what he told me, take it for what it's worth.
He told me that he sells twice as many new .460s as he does new .500s. This was not always the case, but it is now. He also says he takes twice as many .500s in trade as he does .460s. Many times the .500 is traded towards a .460. Seldom is a .460 traded in for a .500. His reasoning is that most that buy a .460 from him, buy it primarily for deer hunting and maybe Black bear. He believes the .460 is a more practical deer hunting cartridge than a .500. We don't have any dangerous game in this area or other animals that need 500-700grain bullets. He also thinks that many of those that bought .500s(especially early on) bought them simply because they are the biggest and the baddest and wanted to impress either themselves or their buddies. Since they are expensive to feed, and kinda overkill on Wisconsin deer, after the initial excitement wore off, they didn't get shot much, so they get traded in or sold. Whereas cost of feeding a .460 is less and the cost of reloading for it is about the same as for a .45LC or .454 Casull. In other words for deer sized game and general range use, the .460 in our area is more practical than the .500. Since there are more used .500s than .460s and demand for them is less(again, in our area) he has to price them accordingly. As for new guns, he claimed that since sales have fallen off on the .500s, that many warehouses and LGSs have new old stock .500s that they bought at the old prices as opposed to the .460s they have in stock that they paid current prices for. Again this is from a small town LGS owner in my area. The prices, demand and practicality may differ in other areas and it could be the owner knows not, but just has an opinion.