Are you talking about how much does cost developer account or how much does it cost to put your app to app market like GooglePlay, Microsoft Shop, AppleStore, maybe Steam?
It depends. If you want to publish your application in market, it is free.
But you should have developer account for this platform.
For example, Android developer account registration is 25$.
iOS Developer Program is 99$/year
There are also other markets - MS store, Steam/etc.
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