A to Z Leak Detection

I want to be more than fair. I should probably give A to Z one star, but because they were pleasant I am giving them two.
When the plumber came to my house he spent about 45 minutes looking into the situation but was unable to do anything other than tell me we had a leak in the main line, but he had no idea where it was. For this, I had to pay $ 250 (a $ 35 «break» from what had been quoted to me after I expressed some unhappiness with the situation).
I am sure this company is fine for locating slab leaks, but I wouldn’t call them if your leak might be on your grounds. Despite their claims, they really don’t have the equipment to find outdoor leaks.
And while the leak specialist was nice, and tried to be accommodating, he was of absolutely no help in the situation. As I told him, I had already pretty much ruled out an interior leak. I had also told the dispatcher/​receptionist that I was fairly sure the leak was in our yard, and that it couldn’t be a pool leak as I manually filled up our pool. If she had told me that based on what I was telling her that A to Z would likely not be able to help me, I never would have agreed to the hefty charge for their coming out.
Parenthetically I need to add that after their leak «specialist» left I called our local water district and explained that judging by the meter a large amount of water was disappearing somewhere. The water district dispatched one of their techs. Even without detection equipment (and A to Z claims they have state of the art), the water district employee was able to determine roughly where the leak was (our front yard), and after turning a shut off valve we were at least able to stop the water. If the A to Z plumber had at least done this, I might have felt better about their visit. As it was, all I could feel was that I had wasted $ 250 for absolutely nothing.
A to Z, if you know you can”t do anything, don’t come out! And if you can’t do anything, is it right to charge $ 250 for less than an hour’s work?