If you go from San luis to Real de Catorce you have to take a Bus (Estrella Blanca) to Matehuala and then take other to Catorce, the cost is about $25 dlls round trip. and 3 and half hours of trip.
Look at the pic, if you travel by car maybe you have to arrive first a la luz town, and there take a nice ride in a truck with many people =), its better this way because you interact with the peolple in a natural way!
Real de Catorce by car...
This cobble stoned tunnel was opened on 2 April 1901 and is about three kilometres long.