Very much; at great cost (6)
I believe the answer is:
dearly
I believe this clue is a double definition.
'very much' is the first definition.
(eg to pay dearly is to pay very much)
'at great cost' is the second definition.
(synonyms)
(Other definitions for dearly that I've seen before include "paying for big mistakes like this!" , "At a high price - with fondness" , "Extremely - devotedly - at great cost" , "With affection - at great cost" , "How I love my wife" .)