Disclaimer about vagueness of memories. This is not exact quoting.
Charlie Brown and Linus get onto the subject of pedigree dogs. Linus says some breeds cost hundreds of dollars apiece. Then he looks at Snoopy and wonders how much his sort would cost. Charlie Brown exclaims, "Hah! His sort are a dime a dozen."
The phrase haunts Snoopy. Over subsequent strips he is seen replaying it, mulling over it. It is quite traumatic. He looks gaunt. He dreams it. Then he comes to the conclusion: "What really got to me was the Hah!".
Linus compounds it by greeting him cheerily as "How's ol' Dime-a-Dozen?".
In one strip, Snoopy's kennel is marked poignantly with its owner's name: OL' DIME-A-DOZEN.
The nearest British version of this is ten a penny.