While the Serbu break-action .50 cal rifle (in collaboration with Royal Nonesuch) is putting .50 BMG sub $1000, I think Safety Harbor Firearms has taken the right approach with their SHTF .50 BMG upper receivers for an AR-15.
The bolt-action upper receivers are offered in either single shot mode or a side-fed magazine version. Barrel lengths available are 18″, 22″ and a long 29″ versions (all 1:15″ twist), all equipped with a massive threaded-on muzzle break to help tame the recoil. For scope mounts, three options are available including “standard high”, “standard low” and “20 MOA high” options for the true long-range aficionados.
The kits use standard AR-15 lower receivers, but they do require some small modification to ensure function. Bolt catch/releases need to be removed and the hammer with corresponding hammer spring need replacement as well.
The operating mechanism is simple. Its a bolt action using two lugs and a longer throw to achieve lockup and handle the bolt thrust off of the large round. Weight is higher than your standard AR, but reasonable at 12.0 lbs (no optic or lower) for the 18″ barrel and up to 15.5 lbs for the long 29″ barrel.
Pricing for the two uppers is reasonable, considering it has the extra machining compared to the single-shot. The single-shot version starts are $1,450 and the magazine fed at $1,850. Lead times from order is about 4-6 weeks.