Yes it would have to be volume to be able to do it so cheaply
When a dive shop in the US markets to a student and then certifies him, the shop is not looking at a one shot deal as that student, once certified will buy dive equipment, dive trips, service there dive equipment purchase dive holidays etc etc.
They have not just certified a diver they have gained a long term customer that will purchase from them time and time again.
They can afford to subsudize certifications to entice people in with low certification prices knowing that that customer will need to buy dive equipement, and that they will become a long term purchaser of there goods and services.
Its a totally different business model compared to a vacation destination who only sees that customer once.
Another thing is with volume of students, some dive shops in the US can afford to employ a full time instructor and make money off him certifying volumes of students where as in the vacation enviroment, the numbers of certifications are much less, its just 1 or 2 students at a time and on a real good day maybe 4.
Instructors are Professionals that have gone through extensive expensive training, and if they take one student over a 4 day period and get paid probably less that 50% of the amount charged, when you divide that to an hourly wage, in many cases with just one student they are working at less than min wage based on the hours they will have put in to that student.
Of course if a vacation instructor was doing consistently 8 students a week, it would be nice, but its just not realistic in Belize.
Belize also has higher costs associated with instructions for example right off the top of that price is GST 12.5% then there is 3% Business tax then the dive shop wants paying for dive equipment $20 a day then the dive shop wants to be paid for 2 days diving $150 then there is the certification card fee then the Padi Manual cost plus any commission paid.
When US shops subsidize they know they will have lots more opportunitys of after sales.
Your never going to get rich as an instructor but the lifestyle can be great I can attest to that