Since it is a beta, and they really need to test the streaming of the rental service, it should be one set price or free for limited time for all beta testers. They can work on pricing/subscription later. EA Access provides a code for a free month.
If it was a small scale closed BETA, sure. Is a little unrealistic to expect things for free on an open BETA though. I also assume this will not be like most BETA's where it goes away and then the final product comes. I assume this will just be a continual service that gets updating and eventually its status will change.