The power costs should be negligible, because even on the machines that are running NXT, only a miniscule fraction of CPU time is needed for NXT. I'd estimate less than 5W on average per "client" node and less than 20W per hallmarked public VPS node for energy. Other stuff is even more negligible, like the opportunity cost of not having as much RAM available with NXT running.

If every public node provider using a VPS stated their monthly hosting costs and we added those up, that would be a number with some significance. Maybe also the network load figures.