Well my rule is what I can afford and what is reasonable...
For example, Glock 23 mags, especially the drop frees are pushing $80-$100. No way I am going to buy a half dozen to dozen of those... Not when the difference is three rounds between those and the nuetered mags. I can get the neutered mags anytime I want for less than $20 per, so why stock up on them?
My Browning hi power mags are pretty cheap, so around 12 for that pistol.
Sig P226, moderately priced so around 6.
Walther P99, couple low caps that came with it and a couple prebans. Now I know that it is legal to use 40 caliber postban mags in a 9mm, I'll be buying some of those as cheap legal high caps...
ARs, around 50. AK47 and AK74, around 25 each. FAL around 25, Cetme around 25, M1A an even dozen I believe.