The fare is a flat rate of 550 yen for adult.
Also there is a Tokyo common bath ticket.
Two children under elementary school age may enter for free with each accompanying adult
Tokyo common bath ticket(10 times tickets) for adult is to be sold.
If you would like to buy it then go to any Sentos in Tokyo(Ticket's expiration date is written on the it).
If the fare changes while expiration date, adjust differencethe balance and will exchange to new one.