The Japanese are famous for their hotsprings obsession and so Tokyo’s full of public bathes (sentos), big and small, private and public, some of which enjoy a steady stream of natural hotsprings (onsens). One of the more up-market posh options for hotsprings in Tokyo and a recommended spot by many recent guidebooks is the LaQau Spa super-onsen which is set next to a theme-park + shopping complex + baseball stadium the Tokyo Dome.
For ~2500yen you can enjoy the hotsprings all day long, with an additional 500yen for the relaxation area on the 7th and 8th floor (worth it if you’re tired and not pressed for time). Considering that it’s Tokyo, It’s not that expensive. Sure, it’s not the China/Taiwan prices I’m used to, but I’d say they’ve set a new standard when it comes to the hotsprings experience.
More about Tokyo Dome and LaQua theme park :