Bookmark Draken is licensed, not sold. No version of Draken is open source or in the public domain. See the EULA for the respective Draken version for the full license conditions.
The Draken models for Prepar3D are currently priced at 9€ each. This includes free updates within each major release series for those models.
Draken 3.2 and 4.1 for Microsoft Flight Simulator are freeware, which means that they can be downloaded and installed without cost. The only requirement is that you register an account on this website. Your account will not be used for any other purposes than for communication with this website. See the Privacy Policy for more information.