Costs and rounding funds, let's see:
You have an android development team, an iOS development team, a backend development team, a web development team, a infrastructure engineering team, a tracking/analytics team, product team to decide features and direction for the app/product and design team to make sure it looks pretty and consistent, then you have a community and support team, and marketing and PR team.
That will probably be the basic. But then of course you need stuff that any normal company need like paper stuff, law stuff, taxes, etc.
All those teams needs to get paid, they need an office to work at, this office need to have water, electricity, computers, internet connection.
The system those teams are creating need to run on some web-service, so certainly there's a lot of money to paid the webservice provider (like Google or Amazon).
But if you really think you can do all this all by yourself, then by all means, the cost is just the $25 that Google Play charges you plus any marketing costs you might want to promote your service.