AFARR
.50's are expensive.
Bottom line:
$1300 Watson Upper for an AR.
Next tier:
$2000-2500 for an LAR Griz, AR-50, State Arms, Barrett 99, etc.
Then:
$4000-5000 for a Barrett 95, McBros, Ramo, etc.
Most expensive:
$7000+ for the Barrett 82 and EDM.
The EDM is probably the most accurate repeater. Of the single shots, it's anyone's guess--I have heard of excellent accuracy from the AR-50's, Ramo's, McBros, State Arms--a lot of it depends on how willing the shooter is to tailor loads (by reloading) to their rifle.
I just bought a State Arms Knob Creek 2000 (haven't shot it yet) and put a deposit on a used Barrett 82 (I figure that way, I have accuracy and firepower covered).
If you think the Guns are expensive--try the rest of the stuff ($1.50 a round+ for ammo, Scopes START at $300 and go up, Bipods for $150, etc).
For an excellent forum, try www.biggerhammer.net and look at their barrett discussion (it covers all the .50's, not just barrett) boards. It has some excellent sources of information--Triggerfifty literally wrote the book on "Hard Target Interdiction", and there are several manufacturer's representatives who freqent the board.