Chapter 16&17 Curiosities

If a product costs $100 and it's on sale for 20% off, then someone may say that he saved $20. However, he walks out with 80 fewer dollars than he previously had, so he spent less. If there a difference between saving $20 and spending $20 less. Also, how do expectations play into this equation? Finally, in the case of spending $20 less, what is the best way to really save $20 (put in the bank, invest, spend on something else, etc.).