About the cost of A PC and an Android phone or a Mac, an Android Phone, an IOS Phone and $100 per year. And some time.
And once you get started, the marginal cost is almost nothing but time if you can do the full thing.
I am a hobby iOS developer, and it is the appearance that suffers the most on my low-end products. If you can do your own artwork then you can get by for about $0+your time.