First, because the supply of civilian-owned machine guns was frozen in 1986[1], a mediocre full-auto gun starts at $10,000.
Second, blanks won't cycle a gun designed to fire actual cartridges. You need a blank firing adapter or a gun made to fire only blanks.
Third, because of economies of scale, blanks are significantly more expensive than real ammo. A few mag dumps would exceed $100.
I have a hard time believing that you and your buddies would get blank firing adapters for your ridiculously expensive machine guns so you could burn money even faster than if you were at the range. If everyone handling the guns is sober and you're in a place where the sound of gunfire is acceptable, why not just... set up some targets? It'd be cheaper, more fun, and more believable.
1. https://en.wikipedia.org/wiki/Firearm_Owners_Protection_Act#...