According to our tests, the brand new beta testing mechanism allows the beta testers to see in Google Play the beta version that you have published instead of the production version.
But the payment is not affected: If the app is a paid one, beta testers will have to pay for it, beta or not.
In other words, beta testers don't get a paid app for free.
So if you want to distribute the app without them paying, then you should distribute it outside of Google Play (email the apk for instance).
Beware that if you use Android Licensing API to protect your app, then you'll need to add the beta testers emails in the list of test accounts in the Google Play console > settings > Gmail accounts with testing access.
