There is no argument that the idiom refers to the obvious explanation that something is cheap and sufficiently abundant to be easily acquired. Knowyourphrase.com explains that the dime, or “dimes” as it was originally called when the coin was introduced as U.S. currency in the 1790s, was actually worth a dime with the dominate metal being silver. The Coinage Act of 1965 changed the composition to 75 percent copper and 25 percent nickel. That debasement could have been the origin of the expression, but there was a reference in the Galveston Daily News in   1866 about peaches being a dime a dozen and pigs being even cheaper. A 1937 reference describes “Smiles being a dime a dozen in the Yankee clubhouse.”