Answer:
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.