50p+ is far too much, 10-20p is reasonable.
Often when I'm alone on a TPE/EMT train going to Lime Street, it is so full that I daren't go point Percy at the porcelain on the train as my seat wouldn't be there when I got back. So it's a bit annoying having to pay at the station as the free toilet at the top of the Wirral line stairs, while free, is seldom open and seems to be reserved for the disabled (who in any case wouldn't pass it as they would use the lift!)
And I hate it when I go overseas and eat &/or drink in a bar or restaurant and then want a Jimmy Riddle in their own carsie only to find a lady sitting outside by a table demanding a 50 cent/Euro-cent tip. Half the time you don't even have a coin small enough.