Got an unexpected issue in production where getting an estimate on a metered taxi product would return a fare with fare_id='None'. I believe this is a bug as it should return no fare or fare_id=null. Returning a 'None' string is very confusing and it very hacky to handle.
Sandbox does not behaves this way and return a fare_id=null
(using product_id=fa89086f-ad68-45b3-a325-ca50f1e8262a)
As per Uber documentation "fare_id" is the string parameter - so values as fare_id='None', fare_id='' or fare_id=null are all valid responses and should be handled by your code.