Do I get the bag of rice that is 10 lbs or 5?
When you're shopping and looking at which commodity to buy based upon price, then you need to look at the tag underneath the food item. For instance, publix and most major grocery stories will include the price/oz on the price tag for that item. This is what you need to look at when deciding what to buy, not the total price.
Bigger is not always better. I was looking at plain white rice the other day and noticed through this method that a bad of 10 lbs was 10% more than a bad of 5 lbs. This is strange, but I noticed it in other areas as well. The canned beans were cheaper than the dry stock, so on and so forth.
Food companies like to trip us up by selling something for cheaper and just changing the volume if food. This helps eleminate this devious practice.
This is also a great way to see how much more you're paying for organic and brand name products. Simply compare the price per ounce for the exact same product whether it be the lessor known brand or your usual premium brand.
For BOGO items you may have to cut the price in half to get at the real price, since they may not change it.