I\'ve looked on SO at a lot of questions regarding this, and they all seem to say that if the app is a paid app, then Alpha and Beta testers will also have to pay to test it
No. I think you've miss interpreted the docs.
For example, if you use In-App-Billing
You can make test purchases
without any charge to the user.
or
You can use a real payment method to test with, but google stops the transaction going through, and you will receive a refund notification after 14 days.
I've developed paid apps. Google does not keep 30% from purchases during your testing.