Range from the cheapest replacement of a toilet float valve £25 (P Flynn based in W7) to the dearest replacement of a float valve £65 (Apostol Apostolov based in W13).
Comparing plumbers estimates for replacing a toilet float valve in the cistern tank gives you an idea of who will be better value but the best way to compare costs is to get actual quotes.
What to do - Get rates and quotes to replace a toilet float valve in W5 by posting a job with HaMuch and we will send it to plumbers in W5.

"Replace float valve in toilet cistern"
W13
"I need to check the Valverde in my toilet but the cistern is in the wall"
W3