This will answer your question :
- Decide Whether your App will be Free or Priced
On Google Play, you can publish apps as free to download or priced.
Free apps can be downloaded by any Android user in Google Play. Paid
apps can be downloaded only by users who are in a country that
supports paid downloads and have registered a form of payment in
Google Play, such as a credit card or Direct Carrier Billing.
Deciding whether you apps will be free or paid is important because,
on Google Play, free apps must remain free.
Once you publish an app as a free app, you cannot change it to being a
priced app. However, you can still sell in-app products and
subscriptions through Google Play's In-app Billing service. If you
publish your app as a priced app, you can change it at any time to be
a free app (but cannot then change it back to priced). You can also
sell in-app products and subscriptions. If your app is be priced, or
if you'll be selling in-app products, you need set up a Google Wallet
Merchant Account before you can publish.