At a former gig, the Prometheus server cost 10% of the total AWS bill in cross-AZ bandwidth charges alone. We were in a single region.
I have long thought it would be interesting to build a highly economical workload that is cross-region, but which only uses a single AZ in each. Inter-region bandwidth costs a little more, but if all your heavy traffic (monitoring etc.) is local you will save a ton. And since your only failover is cross-region, it should be easier to maintain the ability to do so as an organization. Also no extra exposure to whole-region failures :)
I have long thought it would be interesting to build a highly economical workload that is cross-region, but which only uses a single AZ in each. Inter-region bandwidth costs a little more, but if all your heavy traffic (monitoring etc.) is local you will save a ton. And since your only failover is cross-region, it should be easier to maintain the ability to do so as an organization. Also no extra exposure to whole-region failures :)