I've never owned a team, but I've crewed in several series (IMSA, SCCA, NASCAR), and have a friend that was a team owner, and my best advice is to start small, not big. Unless you have a massive amount of money burning a hole in your pocket, or know someone that is, that's the best way. I would also start in a series that manfacurers are interested in, and to me, that means a street stock series like Grand Am Cup, or whatever it is called this month.
And if you go that route, you can sell seats to make a bit of money. It won't pay your entire bill, but helps. Keep in mind that if you want to win, this can still be extremely expensive, even at this level. Especially if you want overall wins. However, I think they are running the B-Spec this year, so that might be an option. You still have to build the car, you still have to test and understand it, you still have to fund your transportation, crew costs, meals, fees, crash damage, consumables etc., etc., etc.
Pro Spec Miata might also be a good option, but you can't rent multiple seats with the same car so it does reduce that ability to make money. But if you pick a series with some type of TV exposure, it does increase you chances of getting some sponsorship dollars. Don't expect someone to fork over a million to slap their name on the side of your Mazda though, you have to be realistic.
If I remember correctly, our yearly bill in the late '90's was near the 6 figure mark (not including the cars), and that was before you had to strip and seam weld the cars, etc. Our first year in NASCAR Dash series was almost half a million in 2000, if that puts this in somewhat of a perspective.
Not trying to talk you out of it, but saying you need to understand the car costs are sometimes the least of your expenses. You need to be a REALLY good businessman to run a successful race team, one that makes money instead of one that drains your bank account and sucks away your will to live! 
