I guess it depends on your work load and also on the your choice of data store. Generally, SQL based storage are costlier on cloud based solution due to the fact that those can be only vertically upgraded whereas no-sql ones are cheaper.
So according to me you should first decide on your choice of data-store, which depends on following factors:
- The type of data; is your data structured or it falls under non-structured category?
- Operations that you will perform on the data. Do you have any transactional use-cases?
- Write/Read pattern; is it a read heavy use case or a write heavy one ?
These factors should help you decide on an appropriate data-store. Each database has its own set of advantages and disadvantages. The trick is to choose one based on your use cases and above mentioned factors.
Hope it helps.