I'm one of the Product Manager on Bitbucket Pipelines. The beta is indeed free, with a limitation on the number of minutes per user per month (starting at 300mins/user/month but that may change during the beta).
We haven't decided on the post-beta pricing yet. The beta will help us understand better how our customers are using it so that we can price it accordingly. We're leaning towards a model that scales well with the number of users on an account.
Let me know if that helps.
Thanks,
Sten Pittet