As a fairly new shop (almost 2 years open) that isn't trying to be a bottom feeder, we charge a flat $100 diag fee (assuming it's not something thats going to take half a day or more, like ripping the interior out of a car to find a short or a water leak etc - then the customer gets a phone call explaining that we need more diag time, or we put the car back together and they pick it up.), with the caveat that if we do the repairs, then we waive the diag fee. The entire purpose, in my mind, is to avoid diagnosing cars for people that have no intentions in having us repair the vehicle. I don't want to diag cars for other people to fix, even if we get paid the diag time only. I want to fix cars. And being a small 2-bay shop, people wasting our time is infinitely more valuable than a bit of diag.