A "normal" price, then, should be the $750, adjusted for inflation. Anything else is just simple gouging.
Not big on "supply-and-demand" economics, are we?
When you have an item and two folks want it (or several thousand folks might want it) and one offers you more money than the other, it is only reasonable to sell it to whomever will pay more for it. Repeat this through thousands of iterations -- thousands of guns changing hands several times each over the course of twenty-five years -- and you'll see a spontaneous, natural development of price points based on scarcity and desirability. This is how almost every item is traded almost everywhere (that's not part of a controlled/planned economy).
Yes, the scarcity is 'artificial,' in that it is caused by a government regulation, but the market's reaction to that scarcity is no more illegitimate than it would be (and is) for any rare, scarce, desirable item.
Look, a first pressing of a Beatles album may sell for tens of thousands of dollars. There's no government regulation preventing more from being made/sold. They're worth more than their weight in vinyl. Is that "gouging?" Do you honestly feel that someone who finds a first edition of the White Album in their attic should refuse to charge more than $3.50 for it -- cause that's what it sold for in 1968? Or maybe $15 because of inflation? Or should they sell it for its perceived value to some potential owner in 2011, which might be $10,000?
Is there any possible way this is different for machine guns, S&W Registered Magnums, or any other limited-availability item that people desire to own? Let's not apply a pejorative term like "gouging." No one is dying because they can't afford a 1928 Thompson. It is a luxury item, "worth" whatever someone will pay to own it.
As for a bubble that will burst? Yup. Also part of supply and demand. The demand for full-autos has somewhat lessened in these tough economic times, and the market price points have come down a bit in the last couple of years. If the registry was reopened, the supply side of that equation would change dramatically, also causing the prices for most MGs to fall.
Most. Not all. A vintage M1 Thompson with paperwork to prove it saw Omaha Beach is still going to be worth a LOT of money because it is very rare that that weapon made it there, survived, and was sold into civilian hands. Is selling such a thing for $40,000 "gouging?" More could be made, but more couldn't be sent into combat on that day in that place. That's a pretty artificial condition. Not really "fair," seems to me. Maybe the owner should do the righteous thing and sell it for the value of its parts.