Cost

Cost as first-class quality attribute; five cost dimensions (build/run/change/downtime/opportunity); architecture style as dominant cost driver; ROI patterns (deployment automation 18×, observability, stability patterns); common cost traps; cost-vs-other-ilities trade-offs

Nines table with money attached; conversion via downtime cost; investment cost per nine; where each nine is earned; antipatterns (tyranny of the nines, MTBF without MTTR, redundancy without isolation); architectural choices that make availability cheap or expensive