Big military powers are in a constant need to improve their weapons otherwise they might lag behind. This means that there must be tons of unused bombs, missiles, etc.. which they didn't use up but have already become outdated. They can't keep stockpiling them, I suppose. Although, They can sell some of them but I highly doubt they can sell the majority of that.
How long do they store them? How much does it cost? (Like compared to its original price)
How much does it cost to get rid of them?