In for a penny in for a pound.

This is about total commitment expressed in old UK money terms. It is when you decide to "go the whole hog" in any situation. It's most likely origin would be the world of investment where people might decide to "bet their shirt" and "go for broke" or perhaps it was an excuse originated by gambling addicts to justify "throwing it all away". The alliteration seems to add some validity to the expression but other proverbs such as "look before you leap" and "only fools rush in" come to mind. A proverb with a bold statement usually has a corresponding one with the exact opposite sentiment. It all boils down to "use your common sense" and "don't believe every thing you are told".