I'm not sure AWS costs are something you can do by remembering a few guidelines, rather than actually assigning an order-of-magnitude estimate to every billable thing and adding it all up.
Like the one that bit me recently was a $0.05/1000 cost per thing, which is easy to translate to $0.00005/thing and then mentally round to $0. That one added up to real money at 10^8 things, and would have been a big problem at 10^9. The cool thing about horizontal scaling is that doing something 10^6 times or 10^9 times is going to feel pretty similar when you do it -- the only difference is the order of magnitude of the bill.
Like the one that bit me recently was a $0.05/1000 cost per thing, which is easy to translate to $0.00005/thing and then mentally round to $0. That one added up to real money at 10^8 things, and would have been a big problem at 10^9. The cool thing about horizontal scaling is that doing something 10^6 times or 10^9 times is going to feel pretty similar when you do it -- the only difference is the order of magnitude of the bill.