The cheapest tire isn't always going to be the cheapest per mile. A company with a large fleet like a taxi operation has the advantage of being able to try many different options to see what might end up being the cheapest overall choice. To get started in the right direction, in the US anyway, there is a tire grading system.
Picture from this site. Not sure what they do, but they had a nice picture and a write up on UTQG.
The UTQG/Uniform Tire Quality Grading system was put into place to solve the issue you speak of. Tire companies could advertise milage warranties and traction levels, but it was difficult to get a clear picture of how one tires wear compared to another.
This system gives tires a rating number (and some letters) to indicate how long the tire is expected to live. The picture of the tire above has a treadwear rating of 400. You could compare that to a terrible tire with a rating of 100, and be reasonably sure that it should last about 4 times as long (100 * 4 = 400). The AA is the traction rating, and important for choosing a tire, but not as important for how long it will last. With some casual tire shopping, I have seen ratings in the 300-700 range.
The treadwear rating isn't perfect, and there are accusations of manufactures trying to "game" the system like they do with car fuel economy ratings. A tire with a 600 rating might not last exactly twice as long as a 300 tire, but it should be a much longer lasting tire. Again, the fleet testing will help find the real winners for a specific application.
Another thing to consider on tires is something called "low rolling resistance". Special compounds and tread patterns are used to make the tire roll easier. This is supposed to save fuel, and make up for the slightly higher price of the tire. These tires are marketed as being "green" or "eco" because less fuel saves the environment.