Earlier quoted context omitted.
The most useful way of thinking of CC debt is how much it INCREASES THE PRICE of what you buy. That nice lunch might cost 20$ if you paid cash, but use your CC and it's really 20$ + the interest you are going to pay. If it's going to take you 4 years to pay off your 20% APR CC debt then that 20$ lunch is really 41$ 50c. PS: That is not to say paying cash for lunch changes the numbers when you are in debt; CC increase…
If you pay it off in full, and get 2% cashback then that lunch cost you $19.60
The cost of that lunch should be $19.60, or less.