2007 Dodge Caliber SE with no A/C option

Expect it to cost WAY more to add it later than if it was purchased with the car, and there will need to be a lot of modifications done. I saw an air conditioning expert at the dealership add AC to a full-size van once, and he said he would never attempt that again, and that would be about the easiest vehicle to work on. Yours will be much harder. It took him over a week to chop up the firewall and install all of the components, and another week to get everything working as well as possible.

There's a lot more to it than just adding all of the parts. The radiator is supposed to be bigger, the front springs are stiffer to support the added weight, and the electrical system may be different. You're much better off selling the car and finding a different one that has factory-installed AC.