You can compare it to Serverless SQL databases: when it’s not in use, it won’t cost anything. Storage will always incur some cost, but no compute.
First, be aware that there are no reservations (reserve plans soon) available. Second, pricing to use items will differ from the regular capacities. Some items may have the same price; others will be calculated at x2 or x3. We have to wait for the official documentation to find out what the actual prices will be.
Right now, we don’t know what the cost will look like, though once the documentation is available, we should have a better idea. Assuming it’s not in some inscrutable units.