In for a penny, in for a pound is an old (British) English expression which relates to taking chances.
It has a few different etymologies, but all of them are crime or gambling related. The idea of the expression is that if you are going to be taking a risk of any kind, you may as well take a big risk, and try to reap the bigger reward.
The thought behind 'in for a penny, in for a pound' is closely related to the notion that you may as well be hanged for stealing a sheep as for stealing a lamb: If the punishment for a crime is similar to the punishment of a more severe crime, it makes sense to commit the more severe crime, and hope you get away with it.
The expression is frequently mis-used in the form of describing punishment out of proportion with the crime: "Tom got 4 years imprisonment for being part of that bank robbery. He was only the getaway driver! I guess it's true what the say: in for a penny, in for a pound."
In gambling, this idiom is used about someone who is certain of winning: If you are sure you are going to win a bet, it makes no sense not to play for the sheep station.
Finally, some times, the expression is used in relation with the stock market, in this case it is a bastardisation of 'look after the pennies, and the pounds will look after themselves': Essentially, the expression takes on the meaning that you should never put any money on an investment deal you aren't willing to back with serious money, because you are throwing your money away.
It is worth noting that this idiom is used in much of the English-speaking world, but it remains 'in for a penny, in for a pound', regardless of which monetary unit is in use in the country.