Any money spent on a cab would be down the drain compared to what you could have spent it on: a NEX ticket in the morning and the hotel price difference from somewhere in the sticks to Shinagawa (or Ueno as earlier suggested - which you can then take the new Keisei skyliner, http://www.keisei.co.jp/keisei/tetud.../us/index.html - if you do that I can suggest the Mitsui Garden Hotel RIGHT across from the station. I put my boss there and he liked it; new hotel in Ueno: http://www.gardenhotels.co.jp/eng/ueno/ ) but there are loads of other hotels near Ueno station.