As the author points out, you might save 2 minutes by not chopping your own onions, but you also had to work extra to pay for the convenience of having the onions pre-chopped - which means more time at work, and less time with your family, friends, or just relaxing and taking care of yourself. So how much did you really save?
While it may just seem like a few dollars here and there, it certainly adds up over time - especially with the amount of "convenience" foods that so many families buy on a regular basis.