in for a penny, in for a pound

In for a penny, in for a pound (= Once you start something, you might as well go all the way with it.) ≅ Взялся за гуж — не говори, что не дюж. ≅ Назвался груздем — полезай в кузов.

Something that you say when you have decided to become very involved in an activity, and to put a lot of money or effort into it)

Example: I've put all my savings into this new venture. In for a penny, in for a pound. (CIDI)

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Cf. I might as well be

Cf.

I might as well be hanged/hung for a sheep as a lamb.

Something that you say when you are going to be punished for something so you decide to do something worse because your punishment will not be any more severe

I'm going to be late for work anyway, so I think I'll go to the shop for a paper. I might as well be hanged for a sheep as a lamb. (CIDI)

Origin
In the past, people who stole lambs were killed, so it was worth stealing something more because there was no worse punishment.