When purchasing fert. do not compare the price per bag. What you are buying is feeding days. not all fert. are even close in comparison. The most important consideration is the soure of N or nitrogen. The lesser priced bags will contain much higher amounts of quick release nitrogen. Causes quick growth that at times are uncontrollable. The best fert. will contain higher amounts of slow release nitrogen that are coated with sulfur and sealed with a sealer.
The difference is like getting a bottle of vitamins and taking the whole bottle at once or taking one a day.