2003 Chevy Malibu A/C blows hot air

Air Conditioning problem
2003 Chevy Malibu Automatic 84,000 miles

Hi! I just purchased an 03 Chevy Malibu and was disappointed when I found out the a/c doesn't work well. The breezy night we test drove it, the a/c was perfectly cool. Now however, on a sunny day, my son and I sweat in the car as the a/c blows warm air. I've made sure the indicator light is on. The temperature gauge is turned to cool. I would like to know if this is a common Malibu problem. Or is it just something that needs to be fixed and what kind of cost will be involved? Thanks for your help!