Yes, there are two different general admission areas at the Monterrey and Mexico City stadiums.
The Cancha GA is on the Field. Tickets cost 650 pesos.
The General tickets are in the far back end of the stadium, in the stands - the worst seats in the house. Tickets cost about 350 pesos, I think.
If you can handle waiting in line all day, then standing for five hours straight and pogoing with 25,000 of your closest friends, Cancha GAs are the only way to go. Even the best sit down tickets will be miles from stage, especially at Estadio Azteca in MC.
See you there