Supply and demand. They used to be $5 each. I bought 30 of them at that price in the early 80's when I owned an original Armalite AR-10 that took them. Up until the SR-25 came out, there wasn't much of a market for them. The HAC-7 is the only gun I can think of that used it.
So back when there wasn't much of a market, they were dirt cheap. The mag ban changed all that quick. Once anyone with a SR-25 realized that the ONLY hi-caps were going to be the waffles, they got bought up (at the very low prices it was easy to do) and now they are gone. Since there aren't any available, the price has shot up. The fact that it goes to a high ticket item like an SR-25 seems to add a few bucks to it as well. If you're willing to spend that kinda dough on a rifle, you'll spend on a magazine as well.
I ran across one of the $5 AR-10 mags I had left over when I sold the Armalite set-up long ago. I put it on the sale forum over at Knight's website and it was sold for $80 in a day.
As a magazine, they really aren't all that hot. They are very thin aluminum, and they break the feed lips pretty easy. Hey, when they were only $5 it didn't matter, but they are now the perfect example of paying WAY TOO MUCH for what you're getting. Alas, that's the way pricing works though. Supply and demand.
Ross