That's a custom made gun. It's a .600 Nitro Express which is used to take down Elephants. Each round is about $40.
"At one time the .600 was the most powerful commercial rifle cartridge in the world. However, that title is now held by the .460 Weatherby Magnum and its 8245 fp of muzzle energy. The .600 was designed to deliver the maximum possible stopping power against elephant under the most difficult and dangerous conditions.
Even professional ivory hunters considered it overpowered for anything but emergency use. It is said that a head shot on an elephant that missed the brain would still knock him down and out for a considerable length of time. John Taylor says up to 30 minutes. Rifles for the .600 usually weighed 16 pounds or over. There were actually two loadings for the cartridge, one at 1950 fps and another at 1850. The lighter load was used in Jeffery made doubles. It is necessary to use the load for which the rifle is regulated or the two barrels won't shoot to the same point of impact"
http://pictures.auctionarms.com/5422154836/c5d3d4a5cd2f1a48c9aa85dce77b3182.jpg?aa=20060811042228