IMHO the 500 Magnum has a limited value. The best scenario would be for hunting very large game - mostly as a back up gun. For those purposes, very few rounds need be fired - mostly for practice and familiarity of course. Yes - $80 a box of 20 rnds; but seldom fired. I do not see any practicality of the 500 in a recreational scenario - unless you have Warren Buffet's $$ - lol. Aside from the shocking recoil and to say you have shot it once- it's not that much recreational fun after the first cylinder full.
The main reason (at least IMO) that people bought a 500 mag was just because they could and because it was the King of the hill. Sometimes guys just want to have the biggest bat in town! I think now that owners are seeing the cost of a box of 20 rounds, the revolvers are mostly relegated as either safe queens or to come out to shoot once every few years at best.
Anyone seriously interested in actually shooting a 500 Mag a lot should either search out a good price on ammo and buy in bulk. The other solution would be to reload for it. Other than those two suggestions, they will have to pay the piper for the ammo - whatever the going price is. BTW, Target Sports currently has 500 Mag ammo in stock for $57 / box. When planning to shoot one it makes more sense to acquire the ammo way ahead of time so you don't get rapped on the price from a LGS.