From personally experience, it's best to become acquainted with the purchase and the installation of various Miata electronic sensors, especially the Oxygen sensor. There are two reasons for this: One, if you have your Miata long enough, you will experience the orange check engine light going on at some point. And that usually happens because of a sensor failure (okay there are some other reasons sometimes too, but work with me on this.)
If you buy the replacement sensor yourself on Ebay, it's usually stupidly cheaper than through a dealership.
Second reason, a lot of the sensors are reasonably easy to install yourself.
Miata.net has a nice article on the importance of the oxygen sensor.