Yeah. Pardon my French but the screens are through the F'n roof expensive: about $515/ea.
link to screens here Thats for the small screens. As crazy as it sounds you'd probably be better off with larger screens, 15'' or so, on the side or front of the cart.
I wonder if it might be a good idea to switch around the business model and be an installer of the media devices and let the shop owner pay for or lease the equipment.
Either way don't be discouraged. The concept is a value added solution that is way ahead of a lot of other "ideas" flying around these days.
Danny