A few answers for this question
Some factors. Population. Desirability to live there. Economic activity. Is it the capital of the country or state? Some users will pay big money if they live there in real life.The price might also depend on what the seller bought it for, their profit. How many users are there from that country? How many cities remain available in a country or are the others available too high in price? Knowing the people in the community might help figure these things out.