You know what the right thing to do is, so go do it. You expected the bank to correct an error that was not in your favor, why should they not expect the same of you? How could you look in the mirror in the morning knowing that your bank showed better values than you?

There may be times when the nuisance of correcting a minor error outweighs the value of the error itself (I wouldn't pick a penny up out of the street and spend my day trying to figure out who dropped it) but this is a simple matter and requires only the same kind of effort you already made for your own benefit.

And, seriously, god's reward? That one bears a little more thought.