Hello Padmahas,
Can you please confirm which type of Cloud SQL you created initially?
MySQL (First or Second Generation) or PostgreSQL?
Billing price of your cloud SQL instance depended on instance type, billing plan, storage or network usage etc. You can get more information at Cloud SQL Price page.
You can check pricing information about second generation MySQL instance in this article.
Second generation instances support “ALWAYS” (On) and “NEVER” (Off) activation policy. With activation policy set to “ALWAYS” mode, the instance is charged for every minute that the instance is running.
As described in my previous post, to optimise cost you can change SQL instance activation policy to “NEVER” mode when not in use.