Startup I was at burned 400k/month average sometimes when buying RI we spent 800k.
Had a couple engineers route the database connections over the public load balancers for a month, that cost 20k in network alone.
AWS is not cheap at scale, period.
I have many stories about this startup. The idea is great. If the startup succeeds it will be in spite of how its being managed technically
Problems come from many areas... like the fact it was in the data management space and the 1500 or so i3's running Cassandra plus the hybrid cloud approach of front end in AWS talking to APIs in GKE which talked to backends in AWS because cool technologies you know.
No architecting was done. Build this and put it in the cloud. The team from RU didn't even know about autoscaling groups and when I tried to bring them in I ran into resistance.
I have many examples of "opportunities to optimize" from this place, needles to say my stress level is much lower not being there.