Not bad, maybe $130/night, I think. The hotel above it (where you stay) is quite a typical Japanese hotel, nothing super-special. But of course the onsen itself is amazing. Though you can get a free shuttle from the ferry, I think I'd also recommend renting a moped or bike to go around the island yourself.

That was the most surprising aspect of Japan that I experienced during my short ten day trip years ago - the relaxed harmonious nature settings. I went there to visit my parents while my dad was on sabbatical in Nagoya and fully expected to get my geek on there and in Tokyo, but ended up doing things like laying on my back in a park in Nara and watching the deer roam.

Can I ask what time of year you went and got the place to yourself?