If you want to figure out exactly where your money is going, next time you buy groceries, make a spreadsheet of every item you bought (keep that receipt!) and calculate price/weight.
One common mistake is buying 12oz packages instead of 16oz packages. They're usually about 5-10% cheaper but are 25% smaller. Obviously if you stop and think about it you know it's not a pound, but if you're in a hurry and your hand grabs that one...
Another common mistake is bone-in meat. Now, bone-in meat is great; the bones add flavor, keep things a little bit more moist, and make great stock. But if you're buying beef ribs for $2.50/lb, probably half that weight goes to bones. On the other hand, you can often buy two skinless chicken breasts, or a whole chicken, for the same price; the whole chicken has bones, sure, but it also has breasts around the same size and a lot more besides.
Another common mistake is food that you think is cheap, but is actually not. Great example: spam. Spam used to be cheap meat. Spam was given for free to WWII soldiers, so when they came back home, they remembered that they were able to eat real meat and knew they could buy it cheap. But if you look at spam in a grocery store today, it's not exactly cheap; I've seen it at over $3/lb! Similarly, I see hotdogs for $1/lb, and I see hotdogs for $3/lb right next to them. People figure hotdogs are cheap but don't notice that they're charging more for processed bits and ends than they are for boneless pork chops.
Another common mistake is not making the most of your food; in other words, waste. Bones should make stock before they're thrown out. Stale bread can be toasted, or thrown in the oven, or crumbled into dishes. Old bananas go into banana bread, not the garbage. It sounds obvious but there are a lot of little things that add up. Once you get good at this, you get creative - for example, when I make pulled pork, the pork fat / water gets used to make rice and it's the best damned rice ever. Also spoilage is in this category; it's good to stock up on things, but not if they're going to spoil; freeze it, eat it, cook it, whatever. Some things are fine to cook just to give them a couple more days in the fridge (grocery stores do it all the time - chicken that's about to expire, but is perfectly fine, gets spit roasted.) On the other hand, there are plenty of things that look spoiled but aren't; hard cheeses grow mold spots sometimes, but you just cut it off and carry on, because that's how cheese works.
Anyways, once you make the speadsheet, you don't even have to come back here to figure out the next step. You'll say "wait, what, we bought that? Damn, that's off the menu."
