A “coin of the realm” means something valued within a given locale, but the term originally meant actual currency issued by the British monarch.
In the US, “two bits” means 25¢, though the US Mint has never produced a 12.5 cent coin. However, Brits have long called a low-value coin a “bit,” and in the US the term was applied to some early Mexican and Spanish coins in circulation which were valued at 1/8 peso, or about 12.5¢ at the time.
“A penny saved is a penny earned” is one of many wise maxims often credited to Benjamin Franklin, though he never actually said it. He came close with “A penny saved is two pence clear,” and “a penny saved is a penny got,” but these frugal notions weren’t original to Franklin; similar wisdom had been printed over a century before.
In both diameter and thickness, a dime is the smallest circulating American coin. Hence, people say that a very quick-maneuvering vehicle can “turn on a dime,” as can a person who changes their own position on a subject quickly. The same idea is invoked by the expression “stop on a dime.”
The coin toss at the beginning of every NFL game (and overtime, if any) uses a special coin made just for the purpose, with particular coins made just for the Super Bowl.
Shortly after dimes were first minted in the US in 1796, “a dime a dozen” was often used as a sale price for everyday products and indicated a good bargain. By 1930, inflation had rendered a dime far less valuable, and this term was first used with a negative connotation of something so common it is nearly useless.
Though now applied to many manufactured items, “mint condition” originally referred to a brand new coin fresh from the mint that made it.