The least expensive way is to make brewed coffee and then chill it.
At the cafe where I'm working this summer, we can't keep up with the demand for iced coffee.
I put the hot coffee into a container and refrigerate it overnight, but the next morning, it's sold in no time. So, during the day, I make the brewed coffee and put it into a container, and let it sit on the counter to cool down before putting it into the refrigerator. I have one in the refrigerator and one sitting out cooling down. Sometimes, it's so hard to keep up with the demand that the coffee is barely cool enough to avoid melting the ice cubes when I pour it into the cups.
The iced coffee is priced 25 cents more than the hot coffee. Plus a lot of people are buying iced coffee with shots of caramel or vanilla in it. I personally don't like iced coffee, but it's nice to see it be such a hot item this summer.
Rose