This is really cool! Am I right in thinking that the cost for running this program is equivalent to all of the dependency execution durations? i.e no busy waiting?

To a first approximation, yes. There is some small cost for the workflow function itself, but as this doesn't wait on responses and only really executes the side effects, it is not that much. Especially, given that this has comparable semantics to the accurate mode (not the express mode) of StepFunctions (which is charged by number of state transitions and not super cheap).

What is the order of the cost of the workflow function?

yep no busy waiting for calls and sleeps within the system. However, 'sideEffects' where you basically just commit the result of an external operation still block the Lambda. But we're thinking about exposing a `fetch` api directly from the runtime (ie, we do a call on your behalf) that could in theory sort that out