I have talked with several recent expatriates who are surprised at the daily cost of living in Mazatlan. It is not expensive by any standard to live here, but it is also not cheap. It's easy to be fooled into thinking Mexico is cheap, especially with the proliferation of travel books out there like "Mexico for $18 a Day". It is possible to live here for $18 a day, but you are probably not going to like it.
I have a cousin who lived in Mexico for a scant $500US a month. He lived in a hut on a remote beach with a few meager possessions: a hammock, a blanket, a pot, a pan, a few plates, cups, and forks, a change of clothes, and that's about it. He didn't even have walls. He fished for his meals. Honestly, most of his budget was beer money. While that may be a great, carefree adventure for a 20-year-old, it's not the way most of us want to live.