You don't make millions of dollars in designing rollercoasters, you basically get paid what you would get paid doing geology, or some other science like that. That money goes towards paying for the steel, the people who make caculations on the ride, the transporters, and the people who build the ride, and other costs.
No, the designer is the one who's going to be paid the most. Do you think it requires the level of education to build the ride as it does to design it? Do you think any of the people who are building the coaster have the knowledge or math skills to be able to design the ride that they're building? Chances are they don't. I don't know how much coaster designers get paid, but the more education you have, the higher you'll be paid, and the engineers on the project are always the ones with the good education.