Cloud Spanner is unnecessarily expensive if you don't have the kind of performance requirements it was designed for.

Looks like Spanner is a relational database. DDB is just a key-value pair. So, is it fair to compare against them or am i missing something?

Why wouldn't it be "fair"?

Because a key-value store is a foundamentally simpler data structure (it's an hash) than a relational database, which tracks the relations between different data types. If you make an advanced use of the key-value store, you have a lot of logic in the application (for example to key management, cascade operations between related data...) which a relational database should do for you. It's not fair because there is a development cost in using the key-value you are ignoring.

Dynamo is not a k-v store.

Apart from the points @dullgiulio mentioned.

DDB -> NoSQL, No Automatic backups, No support for ad-hoc querying, eventual consistency (though you can set to get consistency with few tradeoffs) Spanner DB -> RDBMS, Automatic backups, Enriched SQL, Strong consistency.

Let me if you still think its fair to compare these 2 databases.