What is my return on investment : building real world use cases?

I have been considering learning solidity and the intellectual pursuit does motivate me to build some real-world use case on top of ethereum. I can't help wonder how such endeavours are legitimately monetized (without being scammy)?

Say I build a popular decentralised gambling site? Is it normal in the decentralized world for the developers to keep a cut of the monies gambled? Or if I take an example of a dex, does the person(s) building it and investing their time and effort just charge a fee like your traditional exchange that goes into their wallet?

How do those who build dapps or daos and all that create a revenue flow for themselves? What are the incentives? Is it for pure cred and karma only?