Cost optimization isn't simply "buy Reserved Instances." Pro exams present 4-dimensional matrix problems (workload pattern + commitment model + transient discount + hidden costs) requiring optimal trade-off selection. The same EC2 demands different answers for 24/7 operation vs nightly batches vs traffic spikes; "invisible" costs like NAT Gateway often exceed 30% of total billing. Every decision sits atop the Well-Architected Framework's Cost Optimization pillar with 5 design principles (adopt consumption model, measure efficiency, eliminate data center operational costs, analyze/attribute spending, reduce TCO via managed services)