I like to use Rhino Coat, from Red Rhino. Good stuff. Comes in a 6gal pail, only using 4-6ml of solution per color per car. Nice and vibrant with good cherry fragrances at even 6ml's. So if you think about it, this pail can last anywhere from 3785-5678 washes respectivley. Costing at $118 per pail, we're looking at $.005 per ml for 2-3 cents per color per car. Given, this is running the product lean, but still maintaing "show" for your customers, while staying under the industry average which shows 15-25 cents a car for triple foam.
Just a little input!