For Apps in the F-Droid repo only? That could lead to more intensive debates why one package is included and why another isn’t.
Fraud follows money. That is no argument against it, but be careful when contructing such a system.
I am unsure if many people will use it. I would maybe spend a 1€ per month altogether.
This topic could be relevant to What types of apps are you missing from the FOSS ecosystem?
How much would you spend?