You are right.....found this explanation...BUT the penny, subject to inflation, becomes a dime. Thanks for the correction.
"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]