Why do you want a .50 bmg?
Not that there is anything wrong with getting something just because you think it's cool and fun. But the why might have some bearing on it.
You are NOT going to get match style accuracy out of one unless you get a special one built that shoots match loaded rounds.
That being said, for the money if you just want to have fun and be able to say you have that .50 bmg rifle, the Armalite AR-50 is an excellent platform that is surprisingly accurate, reasonably priced and fun / easy to shoot. It does weigh a lot, is long and is very loud, but that's what makes it nice to shoot.
One of those with a decent optic (around the $2k range give or take) and some decent high quality commercial ammunition should be well inside your budget with plenty left over and will let you have lots of fun at the range.
That being said, if you were wanting something like really bomb proof that could take tons of pressure without blowing up and you wanted to really push the limits and such, it would be worth waiting for the Hoplite Rifles Phalanx which is about a year out. It will be over your budget just for the rifle alone but it will also let you swap several calibers easily.
If you want semi-auto, then I'd say at this point, the Barrett M107-A1 is pretty much the only way to go.
There are other options such as the DTA, Cadex and Accuracy International, rifles, but they are going to be over your budget just for the rifle, in which case, I'd hold out for the Hoplite rifle.
I'd say go with the Armalite AR-50-A1 and maybe something like a used S&B 5-25x56 scope and a really good set of mounting rings and some decent ammo and have fun.