A cheap onsen resort close to Tokyo?

I would like to go on a day trip from Tokyo to either Hakone or Kusatsu to try an onsen.
Do you know a cheap place to stay there (or somewhere else where there are onsens close to Tokyo)? Can I get there without using the JR lines? (how is the cheapest way to go?)

-Emilie

I think the simpliest is to go to a spa center in Tokyo, why not this one Laqua : http://www.tokyo-dome.co.jp/e/laqua/spa.htm which is situated in the center of Tokyo.

There another interesting one at Odaiba, you can read an article here : http://www.travelagewest.com/newsarticle.asp?articleid=3284

Kusatsu is very good but quite far from Tokyo, and it will costs you a lot by train. You can also go there by bus but it will takes about 4h30.