Investing in bargains

Someone pointed out to me years ago that we need to think of bargains in terms of investments. There is one caveat, however: it must involve items that you definitely would buy anyway, and that you are sure you will use before they go obsolete. For example, if a can of beans is normally $1.00, and the grocery store is running a special at 80 cents, that is a 20% return, and you probably will not find that many places in the investment arena. So buy NO MORE of them than you are sure you would have used anyway, and you have made a 20% return on your money.