There is no fee. We are probably referring to the use of a commercial solution like the Radiant Logic Virtual Directory Server, which is a fantastic but not free commercial solution for mapping RDBMS data sources.
You really need to consider two issues:
1. Syncing the user data (username, first name, last name, etc).
2. Validating user credentials (i.e. user name and password)
To address #1, if your users are all in the DB, you can do a bulk import by transforming the SQL to ldif. You can also use the SCIM endpoints to integrate registration (i.e. use POST /users to add new users)
To address #2 you could write an API in front of the DB, and then call that API to validate the credentials.
But net-net, figuring out how to migrate, and keep multiple data sources in sync will end up costing you money, one way or another, if you value your time.