A penny saved is a penny earned
It is as useful to save money that you already have as it is to earn more.
The original form of this proverb used 'got' or 'gained' instead of 'earned'. That is recorded as early as the 17th century, in George Herbert's Outlandish Proverbs, circa 1633:
A penny spar'd is twice got.
The notion appears to have been that, by declining to spend a penny and to save one's money instead, you are a penny up rather than a penny down, hence 'twice got'. Similarly, football teams, who get three points for a win, class games against their nearest rivals in the league table a 'six-pointer'. That's not great arithmetic, but it does make a good proverb.
The current format of the phrase began, with the 'gained' usage, soon afterwards; for example, this piece from Thomas Fuller's, The history of the worthies of England, circa 1661:
By the same proportion that a penny saved is a penny gained, the preserver of books is a Mate for the Compiler of them.
Not much later again and we find a 'got' usage, as in Edward Ravenscroft's Canterbury Guests, 1695:
This I did to prevent expences, for..A penny sav'd, is a penny got.
The first usage of the current form of the phrase is sometimes attribute to Benjamin Franklin. That attribution is without foundation and printed examples began in the 19th century. 'A penny saved is a penny earned' was printed in an edition of the Pall Mall Magazine in September 1899.
See also: the List of Proverbs.