I have been painting for nearly 18 years and have used many different brands in that time. In many cases when you use the lower cost lines of a paint manufacture, such as Omni from PPG or Dimension from Sherwin Williams, you can end up spending more than if you used the top of the line.
As already mentioned, the lower cost comes from a lower quality of toners used to mix the paint. I have sprayed many gallons of GM's Torch Red over the years. Even over gray primer, the top of the line SW usually covers in 2-3 coats. I had to spray the Dimension line of the same color one time. It took 8 coats before I had coverage. Sure, the shop paid 1/2 the money for it, but it took more than twice the material that it would have if I had been allowed to spray the good stuff. The SW paint rep even told me that in most cases, spraying the cheaper line would cost more in the long run.
Im not knocking the cheaper paint, because they sell tons of the stuff and I have seen many slick paint jobs come from the cheaper lines. Just keep in mind, just because it costs less per gallon, that doesnt mean that your total bill will be cheaper. If you are going to go with a cheaper product, most of the big name manufactures all have a lower cost product under a different product name.