When you buy a gun like a 50BMG, you really can't buy something that won't "break the bank." Such large guns require similarly large (or rather high quality) scopes.
The only scope that I know of that I personally recommend is the Nightforce NXS. I have one in a 5.5x22x-56mm Illuminated. They are big and heavy. In fact, it's aluminum tubing is about twice as thick as most other tubes - twice as thick - can you imagine?
I have my NXS mounted on a Remington Sendero SF in .300 Win Mag. - hardly a kicker for such a scope, but I have shot, and seen many other people shoot, this same scope on the 50 BMG at 1000 yard contests with never any problems. In fact, you can see targets very sharply, crisply, and clearly at long ranges. Want to know some inside information? I spoke with the head of Nightforce some time ago - he told me that they had some problems - albeit minor - with the higher magnification line - like the ones like 8x-32x, but never any problems with the 5.5x-22x line and smaller.
I think I bought my scope for about $1,200 about 2 years ago. You can check with the Optic Zone because I'm pretty sure that they sell them. Plus, you don't have to get the 56mm objective line, they do have 50mm lines that are equally bullet proof.
Some people recommend a higher end Leupold for the 50 BMG, but they too cost about the same price, and I have heard of more of them being sent back for repairs, but never (or at least much less often) an Nightforce for repairs.
With a 50 BMG, you really can't skimp out on optics.
Zachary