it's not cheaper! the difference is microscopical! considering the fact that puma upgun cannot kill m8 greyhound (or anything else for that matter) it would be fair if the puma had upgrades too.
Except.. the M8 and the puma have the exact same chance to kill each other... And the stock puma can deal a lot more damage to various infantry than the stock M8 can.
Oh, and it's far more reliable versus tanks than the M8 is.