Try not to use products that mix fertilizer with weed killer or pre-emergent. This is a common mix so you will have to look hard so as to avoid it.
That said, you will also want to pay attention to the release. This is important and it is the main differenc, at least from what I have heard between cheap and quality fertilizers.
A cheap fertilizers will often have a lot of urea (fast acting) nitrogen added to it. There will be a lot of top growth and the nitrogen will be used quickly. If they are not watered in deeply and/or using them during a hot day, then cheap fertilizers will burn the lawn.
Quality fertilizers on the other hand have a higher percentage of slow acting nitrogen that are safer to use and last a lot longer.
Milorganite is clearancing at $5 dollars for a 35# bag across the country right now.
I have heard it's both Walmart and Home Depots. So the cost factor isn't really true if you pay attention to sales and that is how you get your quality fert. It comes down to certain environmental factors for each individual. Both will get you there for your lawn.
But it may be hard understanding how this actually comes into play when you start applying the fertilizer on your lawn.
The easiet way to solve this problem for me has been to ignore the weight of the fertilizer bag and simply divide the amount of nitrogen desired (1.0 lb nitrogen per 1000 sq ft) by the percentage of nitrogen in the bag (26%). When using percentages in calculations, convert the number to its decimal form (for example, 26% = 0.26; 5% = 0.05).
Another common problem involves determining the area that a bag of fertilizer can cover and how many bags are needed to cover large sites. This is mostly based on peoples gut feeling rather than an understanding of the task at hand.
I guess what I'm saying is develop a plan, and then use products appropriate for that plan. Not the reverse.