1
general::
Very inexpensive, as in Their house was a real bargain, dirt cheap. Although the idea dates back to ancient times, the precise expression, literally meaning ‘‘as cheap as dirt," replaced the now obsolete dog cheap. [Early 1800s]
American Heritage Idioms
2
general::
Phrase(s): dirt cheap
extremely cheap. • Buy some more of those plums. They’re dirt cheap. • In Italy, the peaches are dirt cheap.
McGrawhill's American Idioms And Phrasal Verbs