Are local people actually quite wealthy?

After living here for a while, I’ve noticed that people in Bangkok love to eat at quite fancy restaurants. I can understand doing that for a special dinner, but I also see many office workers in Silom and other areas having fancy lunches and expensive coffee. The cafes and restaurants in office areas seem to target locals more than expats or tourists.

At the same time, I often hear that if expats want to live on a budget, they should adopt a "Thai style" lifestyle. However, Thai style doesn’t seem particularly cheap, imo.

Does anyone know how much the average local earns?

Of course, there are always budget-friendly places, and I’ve seen plenty of them.