drop in a bucket

A tiny amount, especially when compared to a much larger one. I'm glad Tony started repaying the money he borrowed from me, but the five dollars he gave me yesterday is just a drop in a bucket compared to what he still owes. We've recruited hundreds of people to help, but it's still a drop in a bucket if we're going to clean up this oil spill in its entirety. Sure, I'd rather someone donate $20 than nothing, but it's just a drop in a bucket when you think of how much money we're trying to raise.