I’m just throwing out a hypothetical, so I may be completely off base: perhaps aws charges high inter-AZ bandwidth prices to keep users from tunneling traffic between availabilities zones to arbitrage lower Internet/egress costs at AZ 1 vs AZ 3.

Outside of my statement above, I do agree that the cost Amazon pays for bandwidth between their sites, has to be practically nothing at their scale/size (and thus they should charge their customers very little for it, especially considering easy-multi AZ is a big differentiator for cloud vs self-hosting / colo). The user above’s dark fiber MRC prices are spot on.