Was dirt-cheap? Hardly! (4,3,5)
I believe the answer is:
cost the earth
'hardly' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'was dirt-cheap?' is the wordplay.
I cannot quite see how this works, but
'dirt' could be 'earth' (I've seen this before) and 'earth' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for cost the earth that I've seen is " proved extremely expensive".)