The fare is a flat rate of 460 yen for adult.
Also there is a Tokyo common bath ticket.

Sento in Tokyo fare

Adult 【More than 12 years old】
Children【More than 6 years old to less than 12 years old(Elementary)】
Pre-school Children【Less than 6 years old(Pre-schoo)】

Two children under elementary school age may enter for free with each accompanying adult

Tokyo common bath ticket

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.