It's worth noting that Amazon showing ads on amazon.com is more like a "slotting fee" or "slotting allowance" than a display ad. Given that you're at amazon.com because you want to buy something, it's no surprise that online slotting fees can be lucrative. But it doesn't mean Amazon has much room to grow that beyond their overall growth.
It probably depends on the value funnel. As you get closer in rellavance to the target the ability to influence ought to be most valuable. I question if they are monetizing the ad value funnel fully.
