vCPU, RAM, storage, and HA doubling.
Cloud SQL bills vCPU, memory, and storage as separate SKUs, plus backups. Two things drive surprises: enabling high availability approximately doubles the compute charges, and storage auto-grows but never shrinks.
Billed: Separate per-vCPU-hour, per-gibibyte-hour, and per-gibibyte-month SKUs. High availability roughly doubles the instance charges.