Also see the concept of mental accounting https://www.investopedia.com/terms/m/mentalaccounting.asp
People tends to treat money differently based on where the money comes from. E.g. you tend not to attend a concert if the ticket was gifted by someone instead of going to concert if you paid $100 for it.
Same thing here with discount. If you keep thinking spending 10K will get you 1500 discount, you tend to overpay for items or buying unnecessary items to reach the 10K with the discount in mind.
This is why it's not useful to count discount as "return".
