Are you asking about the creative designer (a loop followed by a turn, then a spiral followed by a loop) who would sketch out what he thinks would make for a good roller coaster, or are you asking about the engineering team that would take such a rough outline and create the plans for the structure?
For the creative side, it seems that a good understanding of human physiology and psychology are important to make for an enjoyable experience. Both the aspect of maintaining a thrill for the rider while keeping the G forces at levels that don't make people pass out or intentionally sick would need to be balanced. I would think a background in sports science and medicine would be handy, but an intuitive understanding (read years of experience) is probably just as important.
For the structure of a roller coaster, that's more a civil/mechanical/structural engineering side of things. There are large forces and other conditions that need to be balanced with an incredible focus on safety of the people around that structure.
As far as money and entering the field, this is an incredibly narrow field with job availability being more like a pro basketball career than a typical career. Its not like the background training would not be applicable in many other areas, and if its an area of study that interests you and seems appealing its definitely something you may want to pursue. Dream big, but have a realistic plan in place to make sure you are satisfied along the way and can accomodate yourself should your tastes and dreams change.
If you want to learn more in-depth about the subject instead of speculation, find a company or two that does this profession and try contacting them. For students, most people will take a minute or two and provide some insight and advice.
If you really like the video game, you might try Wayne Hodgins who is the futurist for Autodesk (the AutoCAD company) as I remember him referencing it in his speech at a Microsoft event I attended, he told how much his son liked the game and he was a big fan as well. A creative thinker, I think he could be a really great resource.
There was a program about these devices on either the History Channel, or a similar one two or three years ago. It discussed a lot about Roller Coasters, and named designers, etc. if I recall correctly. You might then try to contact one of these design engineers and find out what is required. Sorry, but I can't actually recall more about the show.
There's an excellent game called 'Roller Coaster Tycoon'. I got the original version in a box of cereal, but there's a far more complicated 3-D one now. You can get a feel for the real thing by messing about with it. One of the top real-life coaster designers either created the game or was a consultant for it, and it's quite intricate. Most engineering and physics principles are factored in, such as allowable height, maximum g's on curves, etc.. Most of mine didn't generate much revenue because when people rode them they got sick.
I'll look into that. In the meantime, you'd better get working on a really solid education in structural and mechanical engineering. Some psychology will be useful as well, for deciding what sort of rides people will like enough to pay for repeatedly.