Soda, chips, candy, etc. If I'm gonna buy it, I'm buying the good one. I've tried many different off/store brand peanut butter cups, and they're always fucking horrendous.
Surprisingly, frozen pizza is the opposite. The store brand ones are really good (at least the ones I get at Shaw's and Market Basket). Meanwhile, I've payed much more in order to try out the big name brands. They are usually smaller, lower quality, and taste worse than the store brand ones that cost nearly half as much.