Is there a cost to participate in the Spaces Beta?

You can use the Spaces Beta at no cost.

We want developers to be able to explore the capabilities of the Spaces Beta without concern for cost. We have set quotas in the Ably free tier to be appropriate for Spaces use, so a free account is a great way to try Spaces out. Paid Ably account holders will be eligible for a rebate for quota usage related to the Spaces SDK Beta. 

If you have been using the Spaces Beta with a paid Ably account and would like to see if you are eligible for a rebate please raise a support ticket to initiate the process.