"Entry level" machine guns are still under $4000. I realize, it's still a wad of cash, but it's not like we're talking the $25k necessary for an M16 these days.
I saw a Chauchat go for $1600 or so on Gun Broker a couple months ago (granted, it's a Chauchat, but still it's transferable), and spitfires are also often in the ~$3,000 range. Macs can still be found on occasion around $4k.
The best thing to suggest is to save your money up so you have it all ready to go, and then watch sites such as arms list or sturm like a hawk. "Steal-of-a-deal" guns DO show up there, but you have to be *FAST* (often, well-priced MG's will be sold in under an hour) and have cash in hand.