Given that from the US Civil war up to Desert Storm the average number of rounds fired per enemy dead remained nearly constant at ~100,000, $500 for a practically guaranteed kill is a bargain.
Where are you getting that stat of 100K rounds per kill?
Back in the Civil War, 100K rounds was one heck of a lot of ammo, and they had some hugely expensive engagements in a single day - I have to believe that the RPK in the Civil War was much lower than 100K. The engagement distances were much lower, too.
I can believe it from WWI onwards, when we had belt-fed weapons - I can believe that easily. But pre-belt? I dunno.