You can purchase the bus ticket on arrival. If I were to guess at the taxi fare direct to the hotel after 11pm, I'd say something like Y7,000.

The train/monorail doesn't strike me as a great option unless you take a taxi from either Hamamatsucho (monorail) or Shinagawa (train). If you are determined to do it entirely by public transit, I guess I'd suggest the Keikyu line to Shinagawa (stairs up and then down over the tracks), Yamanote line (stairs down) to Ebisu (escalator to street level then stairs down to subway), and subway to Roppongi (escalator up but then a rather confusing walk through the complex to the hotel). Make sure you have an umbrella, a portion of the walk at Roppongi Hills isn't covered.

I think the taxi from Hamamatsucho would cost more than Y1,500, which is the amount the above poster suggests, maybe more like Y3,000. But I haven't taken this route and could be wrong.