The short answer is that there really isn't a short answer. If cost per bag is the only consideration given, then the results may be disappointing or misdirected. One has to consider what type of controlled-release nitrogen to use as well as how much. This will likely vary depending upon the season and geographical location. Is phosphorus required or perhaps, banned from use? What form of potash is desired and how much? Are micronutrients needed? A soil test is the best way to get answers to these questions.
Then, talk to a reliable source to interpret the test and make a wise product choice.