There is no "cheap" way to add horsepower. That isn't a very good engine as far as upgrades goes. Also, that's a heavy car. You can do the standard upgrades for about $800.00-1000.00 which would be air intake, complete exhaust re-vamp and a computer upgrade. In all practicality, the car is worth considerably less than any upgrades you'd invest in. If it were the V-6 you'd have more to work with.
Search Mazda MX-6 Questions
Mazda MX-6 Experts