Think of a big purchase you are planning to make or have recently made. How much did it cost? How much per year do you think you will save from this purchase, and for how many years will you get these savings? Estimate the present value of the savings, and subtract the cost of the product. Note that it is rare that any purchase will "pay for itself" (e.g., have a positive NPV). But are the savings enough that the product becomes a lot "cheaper" and more worthwhile for you to buy?