Werner Vogels wrote seven laws for cost-conscious architecture. The core: cost is an architecture decision, not an afterthought.
The first law is the most important. Make cost a non-functional requirement. Just as you factor in performance and security during design, factor in cost. Not when the bill arrives, but upfront.
The second law: observe and measure what you spend. You can’t optimise what you don’t measure. This sounds obvious, but most teams don’t know what their system costs per transaction, per user, per feature.
The lesson is broader than cloud architecture. Every system you build has a cost side. Ignore it and you discover that too late.