Like many weapon related purchases; I normally set a price point for an item and keep my eye open for them. As items become available, at or below that price point, and when I have funds available, I buy. It is sort of like buying stocks or any other commodity.

Glock 22, 15 round mags get bought when I find them at $12 or less.
PMAGS, 30 rounds, any color get bought when I find them below $10

Some prices fluctuate more than others, like ammunition, so I am constantly reassessing the price point.

I am thankful for living in a market economy