by the (unit)
For each unit, as of items sold in this manner. How much does this meat cost by the pound?
by the by
Incidentally; on a side note; by the way. An interjection meant to casually introduce or emphasize additional information in the conversation. Sometimes, though less commonly, written as "by the bye." By the by, I remembered where I left my keys, in case you were wondering. Beckett was an amazing playwright. There's a lecture on his work tomorrow, by the by.
By the way; incidentally.