A pretty penny
A considerable profit, or a large sum of money.
'A pretty penny' used to have the variants 'a fine penny', 'a fair penny' etc, but these have fallen by the wayside. All the forms of the expression came into the language in the 18th century and an early example is from a play by the popular playwright Susanna Centlivre, The Man's Bewitch'd, 1710:
Why here may be a pretty Penny towards, if the Devil don't cross it.
That usage isn't exactly definitive but we can be sure that Centlivre was using the expression with its current meaning as she used it again the following year (as 'a fine penny') in another comic play, called Mar-plot. The context of the 1711 usage was a scene where a character was predicting the certain winning of a wager:
I'm like to make a fine Penny on't.