Drink local rum or Belikin beer and it isn't expensive. Especially at happy hours.
I've never considered the food in San Pedro to be really expensive, unless you eat at the high end places. Try The Reef, El Patio, Caramba's, Jam-Bel Jerk. It just depends on what you order. El Divino at Banana Beach has some very good and very affordable options. There's also decent Chinese carry-out (although darned if I know the names of the places). Try El Fogon for local food at decent prices.
I'm only there occasionally, I'm sure others have lots more ideas and suggestions.