If you are eating nothing but PBJ all the time for every meal, yeah, you're going to have a bad time. But when you break it down into nutrient categories, you get a bunch of fat (peanut butter), a bunch of carbs (bread and jelly), and some protein (bread and PB).
This is 90% of what you need. The only thing you're really missing are some veggies and other sources of vitamins. Bear in mind that you don't need to follow the food pyramid for every single meal. I'm LCHF, so I'll often have an egg for breakfast, something high-fat for lunch (like peanut butter and celery, a stir-fry, or nuts and cheese) and a large salad for dinner.
Now as to the costs, your sandwiches are not going to be cheap if you insist on all-natural, organic, free-range peanut butter, jam, and bread. You can easily spend 5x more on PBJ ingredients at Whole Paycheck as opposed to Walmart, but you will not be getting a sandwich that tastes 5x better or is 5x more healthy. The whole point of PBJ is that it is delicious and cheap. If supermarket store-brand products are not your thing, I guess you could make your own bread in a bread machine, process your own peanut butter from bulk nuts, and can your own jelly. But to me, all that work would take the fun out of something that's known for being simple, easy, and cheap. :)