With a dedicated Mongo DB cluster (server-full), you are paying for a certain cluster size, per hour. It doesn't matter if you read or write any data to it. You're paying for a machine with a specific amount of storage capacity and cpu. Use it or lose it.

DynamoDB (considered serverless), you're charged based on the read and write throughput, and how much you have stored (GB-month). If you don't store any data, you're not being charged for unused storage capacity, like a dedicated cluster. You don't think about the instance size, amount of memory, etc.

- https://www.mongodb.com/pricing

- https://aws.amazon.com/dynamodb/pricing/on-demand/