In my opinion keep the app free but have an option to donate funds to keep the project going.
We're trying to build up Nostr and I know I'm trying to onboard new users when I can but telling them they need to pay for an app to use it, will quickly turn them off to the idea.
I agree the main focus is to try and grow Nostr as much as we can and show people the freedom and power of decentralization, FOSS and cool features like zaps.