Spawn is in preview at the moment, so it's free to use right now. However, it is subject to one limitation:
If this limit causes you problems using Spawn, please contact us and we'll see if we can increase it to match what you need.
Once we've got further through the preview we'll be discussing pricing with our users before we announce it, though given that we're a hosted service it's likely to scale with the compute costs we incur running it.
We're hoping to offer a free tier with a set number of minutes per month, similar to GitHub Actions.