I have attached a map of the main bars in the streetwalker area. La Botana is a freelancer bar that is popular with the locals. I don't think it has rooms. Terraza Colonial is right where the streetwalkers hang out and supposedly has rooms for 100 pesos. If you don't see any streetwalkers, look inside the bar. Hotel Montes de Oca has rooms for something like 180 pesos for up to 4 hours. There is often a lone streetwalker in front of the hotel that is better looking than the others. The going rate for the girl is around 500 pesos.
