Monterrey subway system is called Metrorrey.
Tickets are sold with the number of purchased trips encoded. Fare depends on trips acquired:
One trip $4.50 pesos (about 40 US cents)
Thirty $100 (about US$8.70)
Fares since Jan 2003, exchange rate Sep 2004.
See first website below for map (English).
Second one is the official website (Spanish).
The Metro is a very fast vehicle for crossing the city, is cheap and secure. In the tickets machine, if you choose 2 tickets, you not receive 2 tickets, you receive one which contains two travels. When you enter the ticket in the entrance, it marks 'LE QUEDAN 01 VIAJES' (You have 1 trips pending). The same it happens when you choose other number of tickets.