Discounted future /free/ cash flows.

That's cash flow minus capital expenditures which means, at some point, Amazon is going to have to decide enough is enough and allow overall cash flow to come in well above its capital expenditures...like it's just done. Better a bit sooner than later (thanks to the discount part) if you think can sustain your advantage for less than you've been spending in proportion to your revenue.

Perhaps Amazon has reached some sort of cruising altitude.