In for a penny, in for a pound
This expression indicates the intention of seeing a course of action through, regardless of what it may entail. It suggests that, if one is decided to do something, one may as well do it wholeheartedly.
There's not a great deal to be said about the origin of this little phrase. The precise coinage isn't known, although it was in circulation in the late 17th century, as this example from Edward Ravenscroft's comic play Canterbury Guests, 1695 shows:
Well than, O'er shooes, o'er boots. And In for a Penny, in for a Pound.