Quote from a529612:
You mean the paperwork is expensive or the gun itself is hard to get?
Full-auto weapons are extremely scarce on the civilian market, making them very expensive.
The paperwork supposedly isn't all that bad (I haven't done this myself). $200 tax on any NFA weapons including machine guns, silencers, and "destructive devices". You just need to fill out some forms for the ATF and then get a local law enforcement official (only certain positions count, you'd have to check the forms) to authorize it. There's also an FBI background check. You can get around the sign-off and background check by setting up a corporation or trust or other legal entity that is the purchaser of the guns. Once the ATF says you're clear all you have to do is take the paperwork to your Class III firearms dealer and pick out what you want.
EDIT: check out http://www.autoweapons.com/products/products.html some of them have prices listed.