If .50 cal has light AT it is has to cost between 320-400 MP.
If it's any cheaper you could just build 3 50 cals and A-move around the map, killing everything.
But by then raising the price you restrict USF players who just want an anti-infantry HMG.
Solution there would be to give it AP rounds instead of Sprint at vet 1 (and of course, if HMG42 can get an ability for light AT at vet 1 on a 260 MP platform, why can't the 280 MP M2HB get it?). Sprint is definitely still a good ability, but I'd say the niche AP rounds would add to T1 would still be far more impressionable.