Cost of a ring? (4)
I believe the answer is:
toll
I believe this clue is a double definition.
'cost' is the first definition.
(I've seen this before)
'a ring?' is the second definition.
(thesaurus)
'of' is the link.
(Other definitions for toll that I've seen before include "cost" , "Charge for using, eg, road" , "Ring; levy" , "Bridge, tunnel fee" , "Fee paid for use of a public road" .)