San Cristóbal has an enormous selection of Internet cafés, none of which are rubbish, and none of which charge more than 8 pesos / hr. But there's one that stands out way above the rest. Over 40 brand-new PCs. Very high speed Internet. Everything working perfectly. And all for just 5 pesos / hr! The cheapest and the best Internet in Mexico.
I'm not sure what this place is called, or exactly where it is. But it's about 1 block north and half a block west of Posada Mexico Hostel. If you see a large, air-conditioned interior, filled with banks of shiny, numbered terminals, you know you've found the place.