To think in for a penny, in for a pound means that if you have committed to doing something, you should do it completely without sparing any effort, but in this context the person is deliberating carefully.
Consider the staetement "oh hell, I thought if you're going to do something you should deliberate carefully and scrubbed the place from top to bottom". Based on this statement, is "Oh hell, I thought, in for a penny, in for a pound, and scrubbed the place from top to bottom" a true statement? Yes/no/unknown