That's why I'm not a big fan of Metro's currency, it's weird to have 'money' that's also often consumed in game through other actions besides buying things. It creates a weird economy.
... as opposed to having money you spend on bullets which are then consumed in game?
Yes. Money that I can spend on bullets or other gear or whatever. Money that is, as money should be, an abstract concept of value.
Creating a currency that also has a primary function in the game's combat feels bad to me. You essentially force the player to bankrupt themselves in order to stay alive. You end up creating moments when the player regrets doing the ONE THING you've created the game for them to do, kill things. Shooting your gun is negatively reinforced by directly costing the player money. In other games if I get a crap load of ammo I can go nuts with it, go rambo, fuck things up without so much as an inkling of a strategy and just enjoy the mechanics of the weapon. Knowing that even after I waste some ammo it's not going to stop me from using my other money and that my other weapons have enough ammo to sort me out for the foresee-able future.
The concession I'll make is that if your goal is to make the person into a serious gear-miser and constantly worry about their inventory, then I'd take the Metro method. However I still think there are better ways to do this.
Lastly though, if you stick with the Metro method, you box yourself in economy-wise. I can't see a way of having an actual dynamic economy when the player can dump half a town's GDP into a wall inside of a minute. Additionally, you'll run into complications regarding ammo drops around conflict areas and boss room balancing. There's a lot you could do to solve for those issues, but doesn't seem worth it to me.