Amoskeag Auction Company lists a recent sale of a transferrable Browning M2 .50 machine gun for $16, 100. Although it is listed under their Class III auctions, it is most likely a full auto gun which was converted to semi auto (once a firearm is NFA licensed as a Class III weapon, it remains classified as such, even though it is no longer capable of firing full auto). There is one for sale currently on Guns America, which is full auto capable, for $25,995.
Bear in mind, this is for a pre-86, NFA licensed transfer to a civilian buyer, and not what military purchasers pay for them.
45 caliber Machine Gun and 50 caliber machine gun
Depends on the specific model/caliber.
a .45 caliber.
50. caliber machine gun....period
a .45 caliber.
Phosphating
9x19mm
The highest caliber machine gun in standard use is the .50 Cal. Gatling guns are used in 20mm caliber. XXXXXXXXX Russia and China use a heavy machine gun larger than the .50 cal BMG- it is the Vladmirov KDV (for the Russians), in caliber 14.5mm x 115.
1000-100000 USD
See link below to the US Army Field Manual on the Caliber .50 M2 Machine Gun.
50 caliber for US forces
There are several manufacturers who make or have made firearms in that calibre, and cost varies greatly.