Twice as expensive? Oh my! (4,4)
I believe the answer is:
dear dear
'oh my' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'twice as expensive?' is the wordplay.
'twice' shows that letters should be duplicated.
'as expensive?' becomes 'dear' (similar in meaning. I am not sure about the 'as' bit.).
'dear' duplicated is 'DEAR DEAR'.
Can you help me to learn more?
(Other definitions for dear dear that I've seen before include "Expression of dismay" , "That's a pity!" , "Too bad!" .)