When Your Cloud Bill Becomes an Outage

The Lambda function ran perfectly. Every request returned in under 200 ms, the error rate held at 0.02%, and the SLO dashboard glowed green. Then accounting called: last month’s AWS bill had jumped from $340 to $6,200. The service hadn’t failed — it had just quietly bankrupted its budget while meeting every technical metric anyone thought to measure. Traditional site reliability engineering watches three gods: latency, errors, and availability. But modern infrastructure has added a fourth that most teams still treat as an afterthought. Cost doesn’t appear in runbooks or incident channels until someone’s Excel sheet starts screaming. By then, you’re debugging last month’s architecture decisions with this month’s invoice.