I'm convinced that with a good, reliable system, the payoff is short. You need the right tips to deliver a lot of foam (Customers aren't going to keep using something that takes too long to cover their car), a good chemical that puts on a great show and makes a noticeable difference, proper labeling and signage (A name-brand chemical doesn't hurt) and the extra gun in an easy-to-spot location. The mistakes I've seen are: Cheap hose that kinks at the bottom; Chemicals that stain; A tip that makes a thin stream instead of a fan; Poor adjustment that sputters; The gun at the farthest corner from the meter box.
I don't have any system to meter the use of ours, but on a busy day I'll hear it running about half the time. I designed and built the system myself - it's simple and has been 100% reliable. The three pails last about four months.
Think about the payback time: let's say a six bay wash that does $2,000 per bay per month, average use of the tri-foam is 5% of total use. That's $7,200 a year in use alone, not counting the extra rinsing involved.