Dummy Thompson receivers come in wildly differing flavors. Some are just solid aluminum blocks that are good only for a display, and that's all they ever will be. Others, such as those from Bob Bower (Philadelphia Ordnance) and Doug Richardson, are made of steel and could theoretically be made into working guns with some additional machining. But they are not cheap. A Richardson "Ultimax" receiver, with all the options (such as markings), will set you back nearly $2,000. Add a parts kit and you could easily spend $4,000 for a dummy gun. But you would be in good shape if and when the Hughes Amendment is ever repealed.
A West Hurley once went through my hands (when I was an SOT dealer, back in the day), and it was definitely not comparable to my wartime-production gun. By now, though, most Westies have been redone so that they work. (One problem, on some of them, was that the Blish lock ramps were machined at the wrong angle. Fixing that involved welding and re-machining.)
And don't forget that an original Colt-manufactured 21 will be even more. They have achieved cult status.
The aluminum dummy receivers can be convincing, from a distance. I was quite impressed when I first saw one, at a military vehicle convention. But then I looked at it more closely. It wasn't threaded for the barrel, which was just a push fit. So I did some research, and found the Phil Ord and Richardson products. Of those two, the Richardson product is head and shoulders above the Phil Ord. (Don't get me wrong; Bob and Doug are friends.) Doug Richardson is up there in age, and is nearly blind, so it's a question how much longer his products will be available. He's trying to shut down his business and/or find a buyer to carry on.
Well, maybe at first. I think you'd find that most owners of collectible MGs don't shoot them much. It's a function of the age of the collectors, and the age of the guns. This applies doubly to Thompsons.
