In my experience glock mags are more like to $25+. Sometimes less online, usually more like $30 locally. Not a bad deal by any means, but if magpul can make a reliable and widely available mag for $15, there would be some savings.
There are also a fair number of carbines that take glock mags, so they may be going for that market as much as glock pistol shooters.
I run pmags in my ar and have been quite happy with them, so if I bought a 9mm upper, I'd probably be inclined to buy 3 9mm pmags to run in it instead of 2 glock OEM mags.