One of the hallmarks of the Metro series of first-person shooter games, besides their distinctive Russian flavor, is the fact that firearm ammunition is money. In the nuclear wasteland of Metro, bullets are buying power as well as monster deterrent. It makes sense for the narrative, and it creates tension in gameplay. Spend too many rounds on mutant spiders and you may not be able to afford to restock or upgrade your kit.
This is a companion discussion topic for the original entry at http://www.quartertothree.com/fp/2018/02/07/metro-exodus-may-ruin-bullet-economy/