Being starved of ammunition isn't a problem you'll face in Borderlands 2. But if you're like me and happen to spend most of your money on the slot machines in Sanctuary, you'll be happy to know that there are a few ways to save on ammo costs—and one that no one will tell you about.
If you play Salvadore, then you needn't worry about conserving ammo or saving up for slots, as the Gunzerking ability will regenerate your ammunition automatically.
The most obvious tip to save on costs is to open every box you come across. Not only will this work towards your "Pandora's Box" badass rank, it'll also provide you with the ammo you need to keep on killing as you adventure.
The tip no one's going to tell you about (aside from us, that is) is to head back to the starter zone and buy your ammunition there. It actually costs less to buy your ammo from the vending machines in Liar's Berg than it is to do so in Sanctuary.
Another way to save money on ammunition is to equip a Team Ammo Regen class mod. Doing so will regenerate your ammunition and that of your friends for absolutely no cost.
If you've burned all of your money on the slot machines and you find yourself short on ammunition, just kill yourself and Hyperion will spawn you with 33% of your ammunition for every weapon you have, minus grenades and rocket pods. Suicide is a humorously thrifty way of saving money on ammo.
Finally, you could always equip yourself with the Rapid Infinity. It doesn't use ammo.