Since API 38 Salesforce is working on a new kind of object, called Big Object. This kind of record is meant for archiving, tracking and compliance purposes. That is: data that is immutable.
This is a great deal for a number of reasons, but what is really interesting is that this new kind of record is meant for mass storage.
For custom objects we have a rule of thumb that consumes 2kb per record, which means that 500.000 records results in 1gb of storage being used.
How much space does the big object takes on the database?