I think if the government got out of the way there would be a huge explosion in sales but after a while people would figure out they can make their own for pretty cheap. It may not last as long but if you managed to make one for less than a 100 bucks it still would be cheaper than buying a manufactured one.
The explosion in sales was my main point, there just isn't a market to speak of and you are correct, that would change.
As for building your own it isn't really easy at all if you want efficient sound suppression AND accuracy AND reliability all at the same time. Some folks are fine with a threaded adapter and just screwing an oil filter on, but most of us aren't.