Going back to one of my questions that jack answered. My target for rifle mags is normally 10, but that is a minimum and normally just a starting point. For AR's or AK's, I think I have more. For stuff like my Saiga 223 conversion, I stopped at 10 partly due to cost. For pistols, I use the 6 or 8 number. I normally only use 2 or 3 mags at any given time so that leave 3 to 5 as spare. I try to buy only OEM mags, so that can still get expensive. The one mag I regret not buying more was the bakelite 5.45X39 mags. Those fit my AK74 clone the best and were cheap at one time. I haven't seen them under $25 in a while.
I agree that most AR mags are not $10, but if you wait around for sales, you can normally find decent prices. IMO, you get your minimum then sit back and wait for a good deal. If you get on the email lists of a lot of the online dealers, you shouldn't miss the sale.
In my more naive days, I used to buy all my mags at gun shows. Considering the quality I often got and the problems, that wasn't a good idea. Sometimes I knew what I was getting, many times, the mags were just cheap knock offs and were overpriced at that. I think even now I can go to the gun shows in Houston and find a guy selling G3 mags for $20 or more each.