Thanks for the shout, I've made something similar to yours (https://phantasmagoria.stavros.io/) and I needed a GPU backend. Trying out their sample script, it seems to take a minute or so to just error out with "taskID doesn't exist" or similar. Have you hit that issue too?
They're very early in the space. Would recommend looping back with them on Monday in their Discord. Haven't seen that specific issues personally yet though.
Sorry for the late reply: we're running via Banana, they quote half a cent per generation via Stable Diffusion. Rough estimate. Lots of optimization to do though.