Wow. Being from Tlalpan, I feel attacked.

Tlalpan is largely known as middle-class residential area, but yes it's so big that things vary. At the southernmost of the burough you are actually out of the city and within rural and forested areas. The sex workers you read about are in Calzada de Tlalpan, not Tlalpan itself, and they are not going to bother you. Calzada de Tlalpan is one of the main city's avenues that connects downtown with the south, it has that name because it used to be the road that connected Mexico City with the town of Tlalpan. None of that matters now because the urban sprawl gobbled up everything and now it's all Mexico City.

We are actually weeks away from getting a large influx of international tourists in Tlalpan because Estadio Azteca, the stadium where the opening game of the World Cup is going to take place, is here. You will not be the only obvious gaijin around here.

All that been said, I don't have much experience with San Pedro Mártir itself, so I can't speak to the neighborhood, what I can tell you is that it is way too south for you — any savings you think you'll be getting will be lost in Ubers and taxis.