Cost of iron ore, not gold (3)
I believe the answer is:
fee
'cost' is the definition.
(I know that fee is a type of fixed charge)
'iron ore not gold' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'not' indicates named letters should be taken away.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'ore' with 'or' removed is 'e'.
'fe'+'e'='FEE'
'of' acts as a link.
(Other definitions for fee that I've seen before include "Charge for professional services" , "Cost" , "Payment to a professional for services" , "(Entry) charge" , "price" .)