depends on how fast you want to go. i have heard from spec miata guys that it costs $20K + to build a competitive spec miata. for us spec 944 guys, it is closer to $8-9K. that's about what I have in my championship car, and i also had a race win in Utah last year, our unofficial 944spec 'national' championship. on the maintenance front, both cars are about the same, though one our 944 racers switched from spec miata to spec 944 so that people wouldn't be running into him causing body damage every race. we in porsche world race hard but clean. we also help each other out when anyone has problems working to keep everyone running throughout each race weekend.
Ken Huey is the Norcal spec944 director, i know he is putting together an upcoming event for meet and greet and potentially work on building a 944spec car that will act as a guest/loaner car.
Rick