People will pay whatever a school charges, provided that they get goods and services for what they believe is fair market value.
For some localities, the populace may very well be poor, and they believe that fair market value is closer to 30 dollars a month. Maybe a school simply doesn't offer what they're looking for, which would further reinforce a "I don't want to pay more than 30 dollars a month" attitude amongst the people.
Some localities have a wealthier populace, but this isn't necessarily the main reason why people are willing to pay $150 / month. The important thing to remember, is that if a school offers what they're looking for, and if they feel that they are getting what they want, then the amount of money paid for tuition becomes secondary.
We can look at this similarly as we do with cars.
Vehicles will take you from point A to point B. For some people, that's all they're looking for, and they would be perfectly happy buying, say, a $13,000 Ford Focus.
Maybe someone who wants faster performance is willing to spend $20,000 for a sportier car.
Maybe someone who wants to go off-road, is willing to spend $25,000 for an all-wheel drive vehicle.
Maybe someone who wants a luxury car, is willing to spend $50,000 for a Mercedes-Benz.
Any of the above are going to get you from point A to point B, which is the main reason why you bought a vehicle in the first place. For some people, having that GPS unit, leather seats, anti-lock brakes, computerized air conditioning, etc., makes it worth it. For others, they're willing to pay the extra money, since they know that their car is going to be better suited for a particular situation. Some are going to be willing to pay even more, since they've test-driven a particular vehicle, enjoyed what it offered, and are willing to fork over the extra money for those features.
It's not really that different with the martial arts.
Some people are going to be willing to pay $150 / month, because a particular school offers them what they want. Maybe the school has really nice facilities, maybe the school caters to their exact needs better than others do.
Some people are going to be perfectly happy learning out of someone's garage, and paying him 25 bucks a month, even if the garage has rough concrete floors, lacks any amenities, etc.
The bottom line is, that as long as the instruction is good, and the system a reasonably good one, then any of the above individuals are going to get what they want.