How much? That's going to hurt (6)
I believe the answer is:
damage
This clue is a double definition.
'how much?' is the first definition.
(damage can mean cost - what's the damage?)
'that's going to hurt' is the second definition.
(damage does harm or hurts)
(Other definitions for damage that I've seen before include "Ravages" , "Harm, loss" , "To vandalise" , "Deface, harm" , "destruction" .)