What you really want is an ITA Miata.
You have the thought process a bit backwards. In reality, it is less expensive to build an ITA-legal Miata. The Spec rules mandate many parts & changes that you could easily live without. In ITA, the majority of modifications are optional, and if you do decide on a particular option, it does not have to be the exact specified part. A broad example would be shocks. In Spec Miata, you must run the Spec Miata suspension (which includes bilstein shocks, adjustible coilovers, and specific spring rates). In ITA, you could run any spring/shock combination you like, as long as it does not have remote canisters or more than two adjustments. So, in ITA, it would be perfectly legal to run the stock suspension. Keep in mind that the premise of Spec Miata is to provide a standardized platform for competitors. That means that as a racer, you are limited in choice. With an ITA car, you have the option of buying off-brand used parts and such.
I had a '93 Miata that I ran with COM for several years.
At first, the car was run in ST4. At that time, the car had a bolt-in Hard Dog Hard Core M2 roll bar. It was street legal and I could daily drive it. The passenger-side seat mounts were modified to allow a driver-side seat to be bolted in. Both front seats were on driver-side seat sliders. I would drive out to events with a race seat in the passenger side and a stock seat in the driver side. WHen I arrived to the event, I'd swap seats. The car had a full interior and was otherwise prepped and legal for ST4. No problems daily driving or passing Mass inspections, and a heck of a fun track car. I had a small tire trailer that I would tow to/from events. It was really a lot of fun, and in some ways I miss it. Some of the best trips I've been on with COM were to the Canadian events with that car. Having a truck & trailer is great, but there are trade-offs.
A couple of seasons later I decided I wanted a rollcage and to get into SCCA racing, but I would still drive the car to/from events. I purchased a used '99 Miata glass-window convertible top and installed it on the car. This particular glass top can work with rollbars & cages with rear supports. I then took the car to Chris Howard to have a custom cage installed. The cage had to work with the folding soft top, be SCCA legal, and otherwise as tall and as safe as possible. That is exactly what I got. Instead of gutting the doors and running door bars, we went with "x" braces. This allowed me to keep the electric windows and such operational. The result was exactly what I wanted. Same capable and fun car, relatively the same street-driving convenience, more cage, SCCA legal.
Later down the road I used that car at an SCCA school and then a season of races. Many people at the event thought I was a bit nuts for driving to and from the events in that car. I was asked many times "you DO know what happens at these events, right?", this was a reference to car-to-car contact. I was able to keep the car relatively clean during its stint with the SCCA, but it took work. I was aware that I had to drive the car home and had no interest in balling it up, and drove very conservatively. Finishing first would have been awesome, but my goal was to make it out unscathed! It is fun, it is possible, but as all things in life, there are compromises and one must take the good with the bad. I do not have a single regret.
I built another car and let that one sit for a season before selling it to another COM member early this year.
I should also say that my good friend Gordon did pretty much exactly the same with his first track Miata. Both of our cars were developed at the same time and it was a lot of fun and helpful to have two similar cars with similar goals to build and experiment with. The biggest difference between the 2 cars was that mine was a 1.6 and his a 1.8. Other than that, they were both black cars with red tops, and the color choice was purely a coincidence, honest. We used to call them "evil twins".
Both cars are now owned by other COM members who still have them registered and on the road in Massachusetts, yet still know how to make 'em move on the track.
When I bought my current car, I found a deal on a black Miata and then a deal on a red hardtop, so I guess black Miatas and red hardtops must be pretty common. That at least explains why there are 3 of them that run with COM.
In any event, I would suggest you find yourself a 90-95 Miata. These are "ODB-I" cars which will make passing Mass state inspections a lot easier. The 94-95 cars have a slightly bigger engine, slightly bigger brakes, and a better limited-slip differential. The 90-93 cars aren't bad, but the 94-95 cars are just a slight improvement. I wouldn't hesitate to build either.
While it is usually more affordable to purchase an already-built race car, that is not exactly what you are looking for. You are after a street-legal race car, which is almost an oxymoron. Most race cars have removed the airbag system, emission controls, etc. It would be cheaper to buy a street-legal car and make it a race car, then to buy a race car and attempt to make it street legal.
Of course, should either of the black/red Miatas mentioned above come up for sale, JUMP on it. You'd probably have to pry the keys out of their hands though.
By the way, I type extremely fast and seldom spell-check, so apologies in advance.
You guys rock!!! 
