For express workflows, you are charged based on the number of requests for your workflow (at a rate of $1.00 per 1 million requests, or $0.000001 per request) and its duration. Workflow duration is calculated from the time it starts executing until it completes successfully or is terminated. The price per 100 ms for the duration of your workflow depends on the amount of memory consumed during execution. See the Step Functions pricing page for full details of the various pricing tiers for express workflows.