Before the ban manufacturers of affected firearms and mags stepped up production to around the clock, hiring more workers, working 24/7. In just a few months they cranked out enough to keep the market almost supplied for the next 10 years.
There was a gradual rise in price from $8 (average price for New In Wrapper) for USGI mags to as high as $50. As the price of good mags increased, so did the prices of junk mags (USA, used Orlites, etc). The good mags went first, of course, then people began to turn to the junk.
As the end of the ban neared, and it looked like we were going to win (no renewal) the people who had been holding back mags hoping for a continued ban (and even higher prices) began to dump their mags on the market. Prices came down. Current prices are almost to where they would be if there had been no ban, considering inflation. I would expect the price will settle to about $12 and stabilize.
Something else happened. Suddenly there were followers, floorplates, springs available. Before the ban, if a mag did not work right, it was tossed and a new one purchased. But during the ban, mags were babied, maintained, and coddled. Stock replacement springs and other parts were available, as well as improved versions. The new market for spare parts would ensure that the mags that were owned would be kept in like new condition.
For other mags, such as Federal Ordnance aka PMI Mini-14 Mags, PMI went out of business. Hopefully whoever has the tooling will gear up, there is a market waiting to be satisfied there. Those mags will continue to rise as supplies dry up, unless another manufacturer steps in with a comparable product.
This is simple Economics 101. It went exactly as predicted.