After checking the Thai Schools on Sataban.com, it appears that the average tuition fee of International schools in Thailand goes from as low as 100,000 (-ish) Baht a year to 700,000 Baht a year: wwhy such a huge difference of tuition fees among International schools in Thailand?

the bigger the fees the bigger the face for the parents ( and bigger cars for the owners of the school )

LOL..Yes, I am really wondering what is the difference? I have seen some really good international schools for a bit less than 200,000 Baht/year. Anyway, here is the article about the 3 most expensive international schools in Thailand

the bigger the fees the bigger the face for the parents ( and bigger cars for the owners of the school )

The higher the fees, the more exclusive the school is.

This means that only the very wealthy and privileged send their students to the school. Thus, your child might grow up being best friends with the son of extremely wealthy business owners etc, which may increase your child's opportunities later in life. Likewise, while taking your son to football or similar, you might meet and become friends with other parents who are very wealthy and influential. Not to mention of course that the children of the "wealthy elite" have their own culture which will likely rub off on your children (Which you might want, or not, depending on your own values).

In general, the difference between a very good school and the "best" school usually has very little to do with education.

some of the required extracurricular activities for the larger schools seem to be a money generating plan more than for the benefit of the students.

just ask about what is defined as school "camping" trip which is held at a thai resort/