If I put a cold air intake on my 03 cavalier 2.2 ecotech will eat make it run better, and will it reduce gas mileage or increase it???? Also what would a cold air intake cost????

run better yes
increase gas mileage technically (see side note)
cold air name brand 200-250, off brand 75-150

*side note*
if you drive the exact same way you did before itll save you a smidge, most people when they add horsepower drive with a heavier foot, so it seems like you are getting less mpg.