First the round. .50 BMG (Browning Machine Gun), designed about 100 years ago and adopted in 1921, it was a round developed for use in a heavy machine gun.
Now the rifle. A purpose built long distance sniper rifle, designed around the cartridge, taking what was a machine gun round, and using it in a semi-automatic scoped rifle for one shot accurate hits.
This is a rifle that needs a scope. That scope has to give you the visibility to see out at the ranges the rifle is capable of covering and it has to take punishment of the recoil while holding the zero. There's only a few suitable choices and they can add thousands of dollars to the cost of the rifle.
Then there's the range. As Aczarnowski mentioned in his comment, this rifle needs an appropriate range. Anything less than 1000 yards seems cramped. With the mythical uncle's money, a range like the one in this video is possible, allowing the rifle, scope, and that first pallet of ammo to be used as designed.