I have seen a rack with power and 10Gig ethernet advertised for $700 per month; so my guess is that Amazon pays less than 25 cents per Mbps per month and in some cases, nothing.
We can safely assume that almost 100% of their traffic is settlement free peered so obsensibly 'free'. They do however haul the traffic on their own backbone extensively before handing it over (which will be part of the reason for the extremely high level of settlement free). That is extensive and costly ; they lease fibres in subsea cables and in some cases part own them. Similar for terrestrial fibre. This wraps the world, so the fixed cost of that is super high, but the unit costs would be tiny. All in I'd be astonished if the weighted average for say Europe was more than 0.11 cents per GB if they are charging you 2 cents that's ~20x cost, if it's 8 cents...