Are the prices all over the place or are they the same?

Gas or the food things?

Often the prices are all the same because the higher cost ones have gone out of business or adapted to the lower cost method. Price fixing (which is what you're describing) is hard to do when there is a lot of competition, since any one company can disrupt the whole thing and make more money because of it. But it is more likely were competition is scarce. In duopolies or similarly few players, they can price fix more easily. That's why I am in favor of nationalizing industries like that, or at least they need more oversight.