1
general::
Also, by the hundred or thousand. According to a definite quantity, as in She's buying tapes by the dozen. This usage is generally employed for some kind of rate. A 1950 film about efficiency expert Frank Gilbreth and his family was entitled Cheaper by the Dozen. [c. 1300]
American Heritage Idioms
2
general::
Phrase(s): by the dozen
in groups of 12. (Compare this with by the dozens.) • Eggs are normally sold by the dozen.
McGrawhill's American Idioms And Phrasal Verbs