Not sure about the cheapest, but stay away from a vulcan 50 bmg. they have a tendency to put the bolt into the shooter.
Maybe I'm missing something but the 458 Lott is not a range or accuracy cartridge so if you're firing this gun off the bench or down range for kicks or whatever then at least get paid for it otherwise being a breathing crash dummy doesn't make much sense. I worked at a high end gun shop years ago and we took in many like new safari rifles with a couple of boxes of ammo that might have four dimpled primers. The 458's and similar are really made only for heavy thick skinned African game and if you can afford the safari and tags then the cost of a box of ammo is just a days tip for your driver.
Regards
What has hunting got to do with it? I doubt the OP is planning to hunt with his 50 BMG either. Ammo costs to shoot paper or steel whether at 50 or 500 yds are still relevant to overall price of shooting these big bores.
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I was saying that the cost of a precision rifle (in 6.5cm or 50 cal) is only a small part of the whole package, so buying a cheap rifle will only mean you wasted money. A 50 cal system with a good sight, misc shooting accessories (range finder, spotting scope, ammo, sight...) is $6k - $15K. So comparing a cheap $1500 single shot rifle versus nice bolt action rifle at $3000 means only a 10-20% difference in total price.