It may be a bit late for
the subway , there are other alternatives like the
aerocity service , the kind of airport shuttle you find in the States, and there is also an airport bus that will take from the airport into the city center, from which you could either take the tube or a much more economical taxi.
If you take a taxi from the airport to the city center after 11pm expect at least 3000pesetas depending on your suitcases. That will be a good price anyway, keep in mind that it will be around 2500 2700 for a Madrid born parson in the same circunstances.
[ 06-06-2001: Message edited by: MadridMan ]