There are multiple ways Open source projects maintains their costs,
People say they will pay out of their pockets, we can do that as long as the project have very less userbase and low bandwidth usage. Domain costs are almost negligible if your app became big enough and starts to burn a hole in your pocket with hosting costs.
The ways to monetize a free project is,
These are the ways I know, but there might be better ways.
One suggestion for domains is to get one base domain and use subdomains for all your free/os apps, if your app ever got big enough to warrant its own domain, you can start moving in that direction.
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