Obtaining a full auto is as much an exercise in economics as it is in law. Once you know it's legally possible for you to obtain one, you need to find one at a reasonable price (assuming that money is an issue). From what I have researched (ie, browsed the web for prices over the years) it seems to go as follows:
Every 4 years or so, the market for weapons appears to dry up, then all of a sudden, there's a flood of weapons at similar or slightly higher prices. Because it's a niche market, essentially for the rich, not everyone is quick to buy. As a result of inventory not moving, prices drop. All of a sudden, people start buying, supply dwindles, prices rise again. Rinse and repeat every 4 years.
That's why full auto Uzis went from $600-$1200-$3000-$6000
Every 4 years it's like the rule of doubling. The price increases x 2.
Mac-10s for instance. 3-4 years ago, they were in the $2000-3000 range. Now, they're moving up to $4000 range. ~Double.
Machineguns are as much a rich man's toy, as they are a status symbol amongst class 3 owners. It's sort of a way of saying, "look how much money I have to piss away". Same thing with destructive devices (the ones that you have to register each piece of ordnance they shoot). I'm looking into class 3, but I'm looking at the poor man's route- used shotgun, sawed off, sub $200 + $200 tax. The $400 NFA item.