According to this link :
Google has changed it's prices from 11 June. Is that mean using google map SDK for mobile devices is no longer free ?
Yes and no. You'll need an API key and a valid billing method. That being said, the first 200$ (I assume USD) of usage is free
We’ve heard that you want simple, easy to understand pricing that gives you access to all our core APIs. That’s one of the reasons we merged our Standard and Premium plans to form one pay-as-you go pricing plan for our core products. With this new plan, developers will receive the first $200 of monthly usage for free. We estimate that most of you will have monthly usage that will keep you within this free tier.
From https://mapsplatform.googleblog.com/2018/05/introducing-google-maps-platform.html
After that, your credit card will be charged. To estimate your usage you can refer to this pricing table: https://cloud.google.com/maps-platform/pricing/sheet/