In Firebase’s case you will probably be on the free tier (as long as you are not guzzling data) up to 100k users . Think about this for a moment. You will have 100k users on your app. Even if you only add advertising you should be able to make the $25 a month to cover the Firebase FLAME costs.
Read moreIs Firebase email Auth free?
Auth is free . Firebase Authentication service is free in all plans.
Read moreWhat is Firebase Auth?
Firebase Authentication provides backend services, easy-to-use SDKs, and ready-made UI libraries to authenticate users to your app . It supports authentication using passwords, phone numbers, popular federated identity providers like Google, Facebook and Twitter, and more.
Read moreIs Firebase phone Auth free?
All other Firebase Auth features are free to use on all plans .
Read moreHow do I get Firebase Auth?
Get Started with Firebase Authentication on Websites
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