The best, single purchase you can make for ammo storage is a dehumidifier. I'm not talking about one of those little damp-chaser heat sticks, but an honest-to-goodness compressor-driven dehumidifier such as this:

30-Pint Dehumidifier-LD301EL at The Home Depot

My basement humidity runs about 70 percent without the dehumidifier. With it, it stays at 40 percent and moisture and rust are not an issue for me. Yes, they are expensive, but the cost it far outweighed by the knowledge that your firearms and ammo are stored in a safe environment.