One example is bread. I was baking bread the other day, and obviously the cost of the ingredients I put in the loaf are less than the cost of buying a loaf at the supermarket, but that doesn't include the cost of putting the oven on.
Or dry beans vs canned beans; does the cost of boiling the beans actually bring the cost up to be equivalent to canned beans?
I know that everyone's energy costs are different so it's not possible for someone to do the calculations for you, but I've never bothered to do them for my own case because bills I get from the energy company just tell me how much I owe them for the month, not "you put the oven on for 30 minutes on the 17th of June and that cost you X". It sounds like a headache to try calculate how much I pay for energy per meal. But if someone else has done that calculation for themselves I'd be interested to read it and see how it works out. My intuition is that, in general, it's cheaper to make things yourself (e.g. bread or beans like above), but I couldn't say that for sure without calculating, which as I said seems like it would be a pain in the ass.