Regarding pay.. that depends. Entry level will probably be poor. Maybe $35,000 USD / yr for an engineer? Taking kind of a wild guess. That's assuming you're getting in at the ground level like many, doing DAQ. That's in a relatively good series anyway. A lot of low-level or amateur racing you may not get paid at all.
There are many, many young folks who say they want to get into motorsport, and haven't proven themselves yet. I believe the idea is to weed a lot out and get down to people who want to do it because they can handle it, and enjoy it so much they don't mind the pay cut for long hours. Bear in mind, with a BS it shouldn't be hard to get an entry job in any other industry at around $55,000 USD / yr.
Once you have specialized knowledge, the pay can go up substantially. Also depends what series. Good crew chiefs etc are certainly well into the 6-digit category.