A lot depends on the amount of slow-release nitrogen--such ingredients cost more--but are longer-lasting, better for the grass, (and non-burning). And there are different types of slow-release with varying properties and prices. Do you want muriate of potash or potassium sulfate? Urea or ammonium sulfate? Pure soluble--or run of the mill? Large granules, small? Greens-grade--or run-of-the mill with a few rocks, sticks and contaminants. Do you mind if its from Russia? Is it a problem if its rock hard and non-spreadable?
Call the department of agriculture in your state. Ask them how may times your dealer has been cited for short percent prodiamine, short weight bags, for urea that was more like 44 percent, for contamination with sand, foreign chemicals or inert stuff.
A lot depends on your trust and the terms your dealer offers. Brand names will try to keep the quality a bit higher--to protect their reputation. Generic--they don't care.