Here are some examples:
Cheapest - I know that I can save a lot of money buy buying my cooking oil in bulk (large containers) at warehouse stores. I've learned, however, that oils don't store well long term. I am careful to keep my oil in a cool (basement), dark (in a box) area to maximize storage potential. Often, though, I will open some oil and use it for a month or two. As is typical with oil that have been stored for a while, it soon starts to smell rancid and I throw it out. So, in this case, buying large containers of oil just results in waste.
Better - Now I buy cooking oil in small containers. I find that even if it has been stored for a year, I am able to use it fast enough that it doesn't turn rancid.
Cheapest - There are two problems that I have with these giant-but-cheaper containers of Miracle Whip and/or mayonnaise. The first issue is that they contain a lot of oils and behave similarly to stored oils as I've described above. The second issue is that my growing boys use these dressings for sandwich making and they aren't so careful about cross contamination. The result is mustard or pickle bits that end up in the dressing. Yuck! I only had to learn this lesson once. I had a huge mayonnaise container in my fridge and it seemed to last forever (and take a ton of fridge space). I did my best to keep it cross-contamination free, but there was only so much I could do. I couldn't wait to get rid of it.
Better - Now I buy small bottles/containers of these dressings. Once opened, we are able to eat the contents quickly. I personally prefer the squeezable bottles because there isn't any danger of mustard cross-contamination. I also realize that in an potential emergency situation where I don't have electricity, I am much more likely to be able to use the contents of these small bottles quickly without having to worry about refrigeration.
Cheapest - Big bulk jars. Peanut butter contains a lot of oil which has all of the challenges listed with oils above. We eat some, but not enough to keep up with those big jars.
Better - I buy smaller jars and rotate and donate more often in order to avoid rancid peanut butter. Your family might eat peanut butter quickly enough to avoid this issue. We don't.
Cheapest - I originally thought all powdered milk was the same. Boy, was I wrong! I've found that choosing powdered milk is an extremely personal purchase. Some are sweet, some are cheap, and some more closely resemble the milk that you are used to drinking.
Better - I made sure to taste several brands of milk before deciding which brand to store. I also evaluated my powdered milk use to determine how much we would use for drinking and/or baking. I did find one that I really like and I wait for sales, but it isn't as cheap as some powdered milks that are available (and that I like a lot less). I'm always hesitant to recommend one brand, but instead encourage you to find what YOU like before you store a lot of it.
Cheapest - Once I bought a large supply of a no-name brand of chili. It tasted terrible! After trying several cans, we donated the rest to the food bank. I've unfortunately made the same mistake several times.
Better - Now I stick to buying brands that we've already tried, especially if I'm buying a lot.
Cheapest - I mentioned this story once before, so forgive me for the repeat. I found an awesome deal on Kix cereal. I had young kids and was sure they would like it. So, I bought tons. Needless to say, they hated the cereal. So again, after several creative tries to use it, I ended up donating most of it to the food bank.
There is so much wisdom in the current recommendation to store three months of food that you already regularly eat. I once read that kids will starve rather than eat unfamiliar food. It's true that at some point, most of us, even kids, would eat almost anything rather than starve. But I can honestly see my kids resisting and consequently whittling themselves down and compromising their health before they would get to that point.
Better - I try to store food items and recipes that are tried and true. We regularly introduce new foods to our kids, but I don't store those items until I know that they like it. Sometimes that means I have to pay a higher price to get those familiar foods, but it prevents waste.
Cheapest - I thought it would be a good idea to make sure that I had a full garden's worth of seeds for the next year in my storage. It's fantastic to get those after-season deals each year and I thought I would be better prepared for next year. I didn't realize, however, that some seeds have a very limited viability. Onions seeds are a good example. I plant green onion seeds every year. But when I tried to grow onions from those season-old seeds, I think I maybe got a handful of green onions at best. "Bargain" seeds can have similar results.
Better - When an entire harvest is dependent upon the quality of the seeds, it's best to choose name brands and buy in season. One note here: There are some seeds that are easily over wintered. This is an area in which it pays to do a little homework. You can also learn how to collect and preserve your own seeds. This is a great self-reliance skill (that I'm still working on).
You get the idea, right? Though it is good to save money, it's better to be smart. Don't buy bulk, store-brand, or even with a group order (even if it a killer deal) unless it is food that works with your own storage habits and your own family's tastes. Sometimes it's worth it to spend a little extra money.