What are 'Retained for Clones' storage costs?

The “Retained for Clones” cost shown in some storage views in SELECT reflects the difference between the storage amount Snowflake billed you for and the amount attributed to the databases that currently exist.

When a database is cloned, the storage cost for the clone is attributed to the upstream object. If the parent object was to be dropped, the storage cost remains unattributed, not reassigned to the clone. We label this unattributed cost as “Retained for clones” in SELECT.

To investigate further, you can query the TABLE_STORAGE_METRICS view in Snowflake by looking up the clone_group_id for the entries which are incurring RETAINED_FOR_CLONE_BYTES. See here for more details.