I can't think of any big bore ammo (458cal and up) that can be purchased in the under $50/box 20rds area. Even 50BMG ammo is more expensive than that I believe.
Reloading is the only option short of a good friend that reloads the caliber and gives/sells the ammo.
The big bores are not cheap to get started in nor cheap to feed compared to run of the mill 30's and 270's. Reloading dies for some of the big bores can get spendy,,often in the $150 to $200 range. The more exotic, the more expensive. Even the shell holders for some of the odd ones are 'special order' sizes. The really large rounds need a reloading press that can handle the oal cartridge length of these as they are often quite a bit longer than the standards more often seen and loaded.
I priced a set of 450 Rigby Magnum dies....right around $200. Then I lucked into a new/unused set for $50. So there are still deals around.
The big rounds eat alot of powder each shot and commercial bullets are ExpensivE. Normally sold in boxes of 50 or even 25 in the larger sizes to dampen the sticker shock. The 45calibers are still the cheapest of the big bores to load of course in terms of powder, bullet and brass costs.
I do with out dies for reloading for .500 x 3" express in a double rifle using a makeshift arrangement to crimp the bullets. Works fine for that application. Magazine rifles are usually a different story though.
All that said,,nothing quite like them for a range session. With full loads, you don't tend to shoot quite as much each time out so perhaps the cost ends up about the same as the smaller stuff..
