it's really not THAT cheap.
ideally you should try to find a miata thats already built. thats much cheaper. trust me.
as far as startup, you need to get your license, so you have to do schools, and thats not cheap. and you go through tires like crazy. and you need a trailer and a truck, and etc etc etc.
i'd say my dad has probably spent 30 grand, but he built his own car. thats probably 2 years of off-and-on racing. we're on the fourth motor, second tranny, third fender, and second paint job.