I feel this is too open for anyone to answer definitively, and that a good answer varies greatly by the target audience and market.

How much is "decent money"? Can a sole developer make enough money to make his hours worthwhile? (I wish... not so sure.)

What money making methods are you open to: a cost for the app itself, a service subscription, in-app purchases, ads (ad networks?), etc.?

On a mobile app by itself, or as part of a bigger brand / platform?

How _niche_? What is your market? How large is your target audience? Does your target audience spend money on apps? How do they spend it? What conversion rate can you expect?

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