Okay, Using roundish numbers here...
10,000 rounds of .22LR is about $700ish.
10,000,000 would be about $700,000.
And from there...
With a smallish budget of $500 million (considering...), you could stock enough ammo to keep a mostly disarmed population brushed back quite a bit, just using .22s...
At which point the population develops an armored trebuchet, and starts lobbing boulders at your solar array.
Because "collapse" of society won't be starting from scratch. There'll be someone wandering around who geeked on medieval siege weapons.