in for a penny, in for a pound

It concerns you to‥prove what you speak. ‥In for a Penny, in for a Pound.
[1695 E. Ravenscroft Canterbury Guests v. i.]

Sampson‥thought to himself, in for a penny in for a pound, and he fairly drank the witch's health in a cupfull of brandy.
[1815 Scott Guy Mannering III. vii.]

Now, gentlemen, I am not a man who does things by halves. Being in for a penny, I am ready as the saying is to be in for a pound.
[1841 Dickens Old Curiosity Shop II. lxvi.]

‘Do you want to go and have a look, sir?’‥‘Why not?‥In for a penny, in for a pound.’
[1979 P. Niesewand Member of Club viii.]

Have you ever suggested that you take the children away for a few days or even a week—in for a penny, in for a pound—after Christmas or near their birthdays?
[2001 Oldie Nov. 66]

Related to: action and consequence; perseverance; risk

Bibliography of major proverb collections and works cited from modern editions is available here.

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.