The Spaces Pricing page has the following at the top:

"The base rate of a Spaces subscription is $5.00 per month and gives you the ability to create multiple Spaces buckets.

The Spaces subscription includes 250 GiB of data storage (cumulative across all of your buckets). Additional storage beyond this allotment is $0.02 per GiB. If you cancel your subscription by destroying all your buckets, your bill will be prorated hourly."

So I see the mention of monthly cost, and prorated hourly cost. That means that it is not clear to me if $0.02 per GiB is monthly or hourly.