Der Professor have 2 different Versions. Supporter Edition (4.99$) and the Free Version. In this table you can see the difference between the two versions. The revenue of the Supporter Edition go directly back into future projects.
@Desura
Der Professor Free Version
Der Professor Supporter Edition
@itch.io
Der Professor Supporter Edition
@IndieDB
Der Professor Free Version 1.5.0 (FULL CLIENT)
Der Professor Free Version 1.5.6 (PATCH)
@Game Jolt
Der Professor Free Version 1.5.6 (FULL CLIENT)