Building the app is one cost and usually not the largest one over a year. It is worth writing the whole list down before you start, because most of it is small and predictable, and the surprises are avoidable.
Building it
With Buildr this is credits. Free accounts get build credits every day, which is enough to find out whether the idea works. Paid plans add a monthly allowance that rolls over, and top-ups if a month goes long. Costs are shown per message, so the bill is never a surprise at the end.
Store and domain fees
Apple and Google each charge to keep a developer account, billed yearly, and they take a percentage of anything sold inside the app. A web app instead wants a domain name, which is small and annual. Check the stores' current figures directly — they change, and a number in a blog post ages badly.
Running it
If your app stores data or has accounts, it has a backend, and a backend costs something once it is used by more than you. If your app calls an AI model — a chat feature, a summarizer, image generation — you pay per call for as long as people use it. This is the line item that catches people out, because it grows with success rather than with effort.
The cost of changing your mind
The cheapest thing you can do is find out early. A web version shared with twenty people for a week costs almost nothing and answers the question that the store fees, the artwork and the review time are all betting on.