Both the alpha and the beta version is (and will be) free.
The final version is not. But it will be only $79 USD as a one time fee (no subscription) for the full license.
All alpha and beta versions will stop working some time after the final has been released. By then, a trial version will be available for free.
You will not be required to pay anything (now nor later) when downloading alpha or beta versions. But you are of course welcome to buy the final when it is launched.