It depends. Can the price change over time, and do you need to keep track of the price changes?
If you don't need to track the price changes, you can go ahead and insert paper_price and digital_price fields in your Book entity. If you need to track the price changes, you will need these in a separate entity, with one record for the paper_price over a certain period, and one record for the digital price over a certain period.
If a version is free, why not just set price to 0.00?
If you are concerned about future change requests, then use a separate entity. This is the correct way to do this, and will provide far more flexibility for the future.
But it's slightly harder to code, as you have to ensure you create/update all records in your controllers. Ensure relational integrity through the use of database transactions, so that if you get an error updating one table, all the updates are rolled back.
My suggestion of using two separate fields is really just a hack to handle a simple requirement.
