I live in Resistencia, about 350Km away from Asuncion, the capital of Paraguay. And about 1000Km away from Buenos Aires, the capital of Argentina, my own country.
Let's assume I want to go to Miami.
If I search flights originating from my city, most travel websites (Google Flights, Despegar, etc) will try to sell me tickets from Resistencia to Buenos Aires, and then to Miami. This will end up costing me 27.000 Argentine pesos. Usually this includes flights from Aerolinas Argentinas only, as it seems to be the lowest priced airline. The trip to Buenos Aires is usually 2000-2500 pesos for both tickets. So the bulk of the pricing is the international flight.
Instead, if I search for flights to Miami from Asuncion, Paraguay, I also get two flights (usually stopping in Lima or Panama), but the price is actually as low as 12.000 Pesos. There are no flights to Asuncion but it's a 5-6 hour bus ride.
Why is this difference so HUGE? I think maybe because the Aerolineas flight is a direct flight, and the others make a stop somewhere else.
Edit: this is, I think, a diffrent question from How do airlines determine ticket prices? because that one explains about "fine" pricing by airlines, simulations, etc. I don't think this is related, because that fine pricing is usually a few % points. This is a whooping 50% difference. There must be something different going on.