If you want cheap mags, you've got to find a gun that the maker MADE a lot of. A mag vendor won't start up a magazine production line for a few hundred of anything -- they've got to make a lot to be able to sell at a lower price.
I once had a SIG P-210-6. There were no after-market mags to be found that were worth having, and the factory mags were about $90 (this was 10 years ago).
I now have a SIG P226 X Five in .40. Factory mags are about $85, if you can find them, and they hold 14 rounds. Mec-gar makes a replacement that holds 17 rounds, and costs $29. (And, it works in both the standard 226 and the X-Five.) You have to do a little sanding on the top of the baseplate to make them work in the X-Five.
$40 for a mag isn't THAT bad.