I have been developing a GPL GUI application for many years by myself in my free time. I have been approached by a company who want to use my code as part of a propitiatory product. This would require the project being re-licensed under a more permissive license (e.g. LGPL).
In return they will help contribute to its development and have talked about a financial arrangement for the change of license.
How can I estimate what a reasonable and fair financial amount would be this kind of arrangement?
I'm keen to get some additional contributors, but I would lose power in the future to make any other financial arrangements and control over how the project is used.