Q&A

To precisely estimate the cost of diagnosing and repairing your vehicle 'online' may be a bit tricky, but I have worked in the Calif. Smog Testing and Repair field for over 22 years, so think that I can move you in the right direction. First, you need someone to retrieve the Check Engine Code to see if it is effecting your Emmisions. You have an OBD-I car and a sharp tech should be able to retieve the code for 1/2 of shop time ( $40-50 ).
By seeing your CO readings, I would look closely at your Oxygen Sensor(s) and their condition. When was the last time you replaced them? Also your vehicle has a Mass Air Flow Sensor that can still be adjusted, which will lean out the Fuel Mixture and decrease the CO but first you have to make sure the Oxygen Sensors are working well and the car is in 'closed loop' or adjusting the Mass Air Flow Sensor will create chaos. Your car has 2 Oxygen Sensors, so it would be wise to determine if only one is worn out, both or neither. I would NOT recommeend a 'Universal' Oxygen Sensor. They are no longer even legal for CA Smog Repairs.
If you go the estimate page on RepairPal you can find the Oxygen sensor replacement cost for a 1990 300 zx. Be sure to click on the 'Estimate' menu on the home page for RepairPal. Then put in your make, model, zip code, year and service. It is under Exhaust and Emissions. It ranges from $174-$253.

This should get your started. Let me know if you need more help.