"Spec Miata" is the only right answer to this question in my *extremely* biased opinion!
Seriously, I still LOVE my Porsche and love driving it on the track, but I get 100% full racing satisfaction out of racing my little girly race car. If you want to do it on a budget, it won't cost you nearly $1k per weekend. If you want to be super competitive, then you'll spend more. You can buy a good, fully prepared, race ready, no frills car WITH TRAILER for $10-12k. Tires, pump gas, trailer and motel (you can usually camp at the track, too) and entry fee and you're in! Parts are REALLY REALLY cheap. You can usually buy an entire spare parts car for $500-700 if you're savvy. Blow an engine? Buy another one for $300 at a junk yard and your track buddies will help you change it out in an afternoon...
I wouldn't be racing if I only considered racing a Porsche (due to financial constraints) so trying something different was the best car decision I ever made.
Check out
www.specmiata.com
Cheers,
Dean