During the 1994-2004 ban prices depended on the magazine. Mags for AR's, Beretta, S&W, Sig, Glock, and Ruger's in 9mm were selling for $30-$50. These were selling for $15-$30 pripr to the ban. They were a bit pricey, but easily available because there were so many out there. A lot of folks found that you could buy a 10 round mag for 40 cal and it would hold 14 rounds of 9mm and function perfectly in theses guns as well.
Guns like the Glock 20,21,22, and 23 had only been introduced a short time prior to the ban and only a handful of those were available. I sold several G23 mags for $100 each. Sold the gun with 2 mags used for $700 right after the ban went in effect. I saw 15 round G20 mags bring $150.