Exactly as Con said, a CZ in 500 J will be the cheapest option, a second-hand #1 you might get for $800-00 to $1000-00 perhaps, bear in mind that an MAB 510 barrel will cost you about double what a normal calibre will, remember getting a 510 barrel from the US might also be out of the question, being the same diameter as a 50 BMG might preclude you from importing one, or getting one imported.
Got a mate that bought a CZ in 500 J, one really important thing to remember with them is that the throat is ridiculously long, we did a chamber cast, and it measures at least an inch in length, way too long in my opinion.
He found out the hard way that only loads that pushed a 535 grainer at 2400 fps would fireform correctly, anything lighter failed to form, or even fire at all. Theory is that the gases tended to push past the projectile while it was in the freebore area. Not a nice problem to have at all, and Yes, we tried seating 600 Woodleighs out as far as possible to hold the case back, and to jam the bullet into the rifling, couldn't do it, the throat/freebore was longer than that. He ended up using a Winchester Australia recommended load of 130 grains of Win 760 behind a 535 Woodleigh, they do around 2400 fps and according to Quickload, that load is in the "purple" region for pressure limits. But that's all that seems to work, other than Norma factory ammo, but at $300-00 per 10, it's a bit dear to shoot.

In my opinion, get a Ruger #1 in any calibre, cry at paying the money for an MAB barrel, and build your own. then you have to think about cases and reloading dies as well, they are a little dearer than 243 cases and dies!

Regards,
David Commens.