Join a military branch of your choice...
But in all reality your looking at about 3k for a MAC or "mac style" full auto SMG. Even at that you'd almost have to get in line for a lage upper to have more than what most would call a "bullet hose" so 4k is about the bare minimum price of full auto admission, plus ammo. From there you just go up and up.
Only positive is right now prices seem to be stable / slightly suppressed. Stuff on subguns isn't going sold in 10 minutes and I've seen private parties dropping asking prices because of financial concerns.