I think the response has been muted because you present a very difficult, somewhat conflicting scenario. But, knowing very little about you (or anything else), I'll wade in even though I am not an expert on all things Panama:
- You want "affordable" housing based on a "modest" source of income. This is tough to assess because your idea and my idea of "affordable" and "modest" may not be close. The fact that you would like or need to supplement your income (see below) tells me that you're may have a $1,500 - $2,000 a month budget. Less than that and you're not "living" you're just "existing," in my opinion. This income range would be the minimum I think based on your age (not old) and dependent.
- Generally the Panama interior offers the cheapest living but requires you to "live like a Panamanian." This is not a derogatory reference, but means to most people a small, simply and sparsely furnished house with a water reserve tank and no A/C. It offers function only and is likely situated in a Panamanian community (verses an expat community). This may be near the Pacific but not in or around a beachfront development or community, in my opinion.
- The more you move down the scale on "affordability," the less education options are presented. You don't state that you and your teenager are fluent in Spanish so I assume you are not. So, "affordability" conflicts a bit with decent education options as well as availability of activities and social life every teenager I've raised or known expects - which are nonexistent in many of the "affordable" areas, again depending on your definition. Some may disagree, but once again it depends on how important quality education is to you and your teenager. Put it this way, no one moves to Panama because of the quality of their education system.
- I think you need to address the needs of your teenager first (education and social life) because this will restrict your living options far more than finding an affordable, cheap place to live somewhere near the Pacific.
- Supplementing your income as a teacher probably should be ruled out. Employment by expats is not allowed and even if permitted under some the newer, vague provisions applicable to "professionals" you will not find the process easy, cheap or timely (that's the Panama way). And, if you do secure employment, your pay will be less than 25% of the salary a similar position would generate in the U.S. or Canada, more or less. The average wage in Panama is around $500 a month and the hours are very long to earn that.
Good luck.