There are three types of purchasing:
1. Buy it when you see it, and don't worry about price;
2. Shop and shop and shop for the lowest possible price, and only purchase the absolute lowest;
3. Work a budget for items that includes reasonable purchase price ranges, and purchase when within that range.
I personally don't have the finances to do #1, and I don't have the time and patience to do #2. However, number three, while not giving me the lowest cost at all times, does fit my lifestyle of assuring that what I purchase is affordable for me, and that it fits within my terms of reasonable cost.
It is true that two tenths of a cent does equate to ten dollars on a case of 5,000 primers, but unless you are already at the store, or in the neighborhood, you can't drive any distance to save $10, especially when you are talking a lot of 5,000 pieces.