be a drop in the bucket

To be a tiny or insignificant amount, especially in comparison to some larger figure or sum. 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 the bucket compared to what he still owes. We've recruited hundreds of people to help, but it's still a drop in the 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 the bucket when you think of how much money we're trying to raise.