What do we have to pay? They've been charged a hell of a lot! (8)
I believe the answer is:
billions
'a hell of a lot' is the definition.
(large amounts of something)
'what do we have to pay? they've been charged' is the wordplay.
'what do we have to pay?' becomes 'bill' (a bill must be paid).
'they've been charged' becomes 'ions' (electrically charged particles).
'bill'+'ions'='BILLIONS'
(Other definitions for billions that I've seen before include "No bills I turn into vast sums" , "Millions of millions" .)