The first .50BMG to be banned would certainly be the semiauto ones. The only one I am currently familiar with, and the one that gets all the media attention, is the Barrett. If I could come up with $7500 right now, I'd be buying one.
The rest of the market is dominated by single shot and repeating (they have a magazine) bolt guns. The Armalite AR-50 is probably the most common. It is a high quality single shot, with a street price (in used but good condition) of $2,000 to $3,000 depending on accessories. The bare rifle will sell for the low end, and one with bipod, a good scope and mounts, case, etc., will sell for $3k or even higher.
You can find decent .50's for as little as $1,000 if you look hard, and they go WAY up depending on make and model, accessories, etc. I really like the Cobb FA-50T, which is a straight-pull bolt action repeater. They are rare, though, and cost in the range of $5-7,000. McMillan makes a highly regarded bolt repeater, also, but again, quality costs $$. A McMillan will set you pack $5k+.