[Disclosure: I am a Google employee and the Product Manager for Cloud SQL]
Every Cloud SQL instance is regionally available and includes seven days of free backups. Data is automatically encrypted and replicated in many geographic locations and failover between copies is handled automatically. Competitors often charge separately for these features (often 2x for automatic failover). Additionally, you will not be charged for your Cloud SQL instances when your database is inactive. A database instance is inactive if there are no connections for 15 minutes for "Per Use" billing and 12 hours for "Package" billing. "Per Use" and "Package" billing options help users find savings based on their database usage. Lastly, Cloud SQL pricing does not require a yearly commitment.
Put together, Cloud SQL users with small, infrequently accessed databases often pay less than $1.00 per month, while users running large, frequently accessed databases pay more. Factoring in features like automatic failover, backups, and no upfront commitment, you will likely find Cloud SQL to be less expensive than competitors for some scenarios and more expensive than competitors for other scenarios.