The standard guidance shows “cost_option” as a UK extension to the base international Open Referral standard which just has a single “fees” text field for services. Cost_option is shown as a new table in blue in the data structure diagram.
As discussed with @pbrown, Placecube has implemented cost_option in its Open Place Directory product but for now it has made cost_option.amount a text field to allow for historic values such as “Free”.
I think amount should remain numeric, as specified and amount_description be used for any text descriptions.
Views from any other implementers are welcome.