It's twice as expensive. What a pity! (4,4)
I believe the answer is:
dear dear
'a pity' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'it's twice as expensive' is the wordplay.
'it's twice' says to double the letters.
'as expensive' becomes 'dear' (similar in meaning. I am not sure about the 'as' bit.).
'dear' doubled is 'DEAR DEAR'.
'what' acts as a link.
Can you help me to learn more?
(Other definitions for dear dear that I've seen before include "That's a pity!" , "Too bad!" , "Expression of dismay" .)