Economical and racing should never be used in the same sentence.
As you're not new to racing I'm sure you already know that though. I've been road racing with a couple different clubs for several years now and although I'm biased as I'm a Spec Miata driver I don't think you can beat the Spec Miata class.
Mazda sponsors the class which means you can buy parts at a much lower cost than most of the other classes. The cars are VERY reliable when compared to most of the other classes. To run a full schedule only requires a couple sets of tires, maybe 2 sets of front pads and rotors and one set of rears. All of which are very reasonable when compared to a lot of other classes. These along with other reasons are why the Spec Miata class is very healthy and growing. You're always guarenteed plenty of competition no matter where or when you race.
That being said, if you want to run at the pointy end of the pack you'll have to spend a lot more. You'll need a pro built motor, dyno tunning, rain tires, and more tires. That pretty much goes for any class though.
I just finished building a ST1 car which has to be at the other end of the spectrum.