in for a penny, in for a pound
Once involved, one must not stop at half-measures. For example, All right, I'll drive you all the way there—in for a penny, in for a pound. This term originally meant that if one owes a penny one might as well owe a pound, and came into American use without changing the British monetary unit to dollar. [Late 1600s] For a synonym, see hanged for a sheep.