Price one's willing to pay rotten, sovereign (5)
I believe the answer is:
offer
'price one's willing to pay' is the definition.
(offer to buy something)
'rotten sovereign' is the wordplay.
'rotten' becomes 'off' (rotten food is 'off').
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'off'+'er'='OFFER'
(Other definitions for offer that I've seen before include "Provide; reduced price" , "Chance to buy at reduced price?" , "Express willingness - present" , "Make available for sale" , "Submit" .)