I don't do a once a month cooking, but I am shopping now for two weeks at a time instead of one as well as trying to make some things to have in the freezer such as muffins, pancakes, extra cooked hamburger.
I have noticed that if I shop for two weeks at once, I do save a little money. Even if it's just $50/month, that's quite a bit a year. I haven't been doing it long enough to gauge a whole year but we will see.
Also, we don't use our microwave a whole lot either for cooking. However, if you put out a meal from the freezer, the night before, it shouldn't take as long in the oven and then you wouldn't have the oven on for quite as much time.
The only other thing I have noticed with 1x month cooking is that with all those meals in the freezer, it's a pretty strict menu...I much prefer having a well stocked freezer and pantry, so I have a lot more options depending on what we feel like eating any particular night.