Well, to get your app published on Google Play Store you have to pay a sum of $25 USDs as it is the registration fee charged by Google. This is a one-time fee, which allows you to have a developer account and through that, a developer can publish as many applications as he wants until you maintain quality. If an app's quality is poor, Google has all rights reserved to suspend your account. Therefore, quality is essential.
As far as publishing your app on iTunes (Apple’s App Store) is concerned, the developer fee can range from free to matching the Apple App Store fee of $99 per year. You’ll be provided with an iTunes Connect account and if you wish to create and publish multiple apps you can do it through using the single account. It is worth noting that the process of app submission on iTunes is very rigid as well as annoying.
The sum of $99 does not change with how many apps you publish on iTunes at a particular time or using the same account for publishing a number of apps. Apart from this, you will have to pay the thirty percent of all sales to whichever platform you choose regardless of whether you are getting any profit from these apps or not.
Besides, there is another platform with having comparatively less popularity named Microsoft Windows, which is extremely cheaper than these platforms. You just have to pay an amount of $12 to publish your app and once it's done, you are on!
The accounts you create with any of the aforementioned platforms, you will be informed about the payment, deductions, charges, etc. through notifications.