Either you have an existing db running and check the metrics, or you don't have a workload to move and you can just try it out from scratch. You can always migrate away if the billing model doesn't work for you.

Alternatively, it's likely that Planetscale will eventually provide a provisioned billing model, similar to DynamoDB. Then you can choose between scalable usage based pricing or less flexible flat rate pricing. Either way, innovating different billing models for RDBMS is a win.