Normally, you get what you pay for. However, I do think some feeds may be overpriced.
I think most Purina feeds are pretty good quality. The only way to know for sure is to get a calculated nutrient profile from the manufacturer. The ingredients and guaranteed analysis may look similar but there are bound to be differences.
Manufacturers aren't required to list anything on the analysis other than crude protein, lysine, methionine, fat, fiber, salt, calcium and phosphorus. There are other important nutrients that the companies assay for and they should be able to give you those numbers if you call them and ask.
Then you can make an educated decision. Pay special attention to vitamins A, D and E as well as amino acids like cystine and arginine.
I have found different feeds to vary dramatically in those nutrients. That's why regular layer feed may be adequate for making table eggs but not for hatchability.
Look for vitamin A to be around 5,000 i.u. or greater, D at 2,000 + and E at 50 i.u. per pound.
Also, always look for the freshest feed.