The phone books and mags they leave in your room are a good first step. Next up would be asking the concierge as you hand him some cash (worked at the Hotel Victoria).
The last time I was there they were still using francs, and it was 500-600 ff for full service (around $150 if I remember correctly). Less expensive if you don't mind old or ugly.
If your friends are bored be sure they hit The Loving Chair in Pigalle (google it to see what it's about -- there's no "service" there, just a unique live sex show). If you go on a weekday afternoon you may be the only person in there.
Paris is a fun place and I plan to return many times.
