I think it depends on the ammo you have. AP, API, SLAP, tracer, Ball. You should be able to get a good idea from a couple of searches on Gunbroker or maybe Ammoseek. I think the first thing is to figure out what you have. I would take a guess and say plain ball ammo. Right now it looks like 2.50 to 3.00 a round for basic linked/unlinked ball ammo.
Are these handloads or factory/GI rounds? I personally wont buy unknown reloaded .50 BMG to shoot, I would buy it for components to reload myself. A bad handload in .5.56 or 7.62N can hurt you but a bad .50 BMG load can kill you, its about like a stick of Dynamite going off next to your head!
Do the bullets have a color code on the tip? Black- AP, orange or red- Tracer, silver- AP/ incendiary, blue- Incendiary, Red And Silver- APIT and there are others. There is some Yellow tip that is AP from the Dom Rep that is dangerous to shoot Period! There are some loaded .50BMG with the projectile from the semi auto gun on the 105mm recoilless rifle, they have a 1000gn bullet with a Yellow and Red code on the tip that can go off in the barrel if loaded to fast, a company named "Talon" loaded a bunch of in the late 90's you still find on the market now and then..