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)