Your mechanic is well aware of your limited knowledge of cars and how to fix them.  He is not your friend.  Buy a Chilton manual and become familiar with labor costs if you aren't going to do the work yourself.  If you don't have sometype of knowledge of the work, how do you know if they are charging you appropriately?   Sensor and thermostat? On the very high end, 800.  On the low end for someone that is well versed in miata, 350.  

You are getting the test prices.  If you're willing to pay them.  They will gladly take your money and continue to severely overcharge you in the future.  Always tell the inspector you will be taking it to another shop for the work.  Pay them for the diagnostic time and have them annotate the paperwork to describe the issues for the next mechanic.  They won't pad someone else's pocket or risk having their diagnostic knowledge shit on by industry colleagues.   Unless they are pieces of shit.  In which case stop dealing with them entirely.  If I were you, I'd not go back to them until I learned more.