For the huge factor in price difference you can keep spare spot VMs on GCP idle and warm all the time and still be an order of magnitude cheaper. You have more features and flexibility with these. You can also discard them at will, they are not charged per month. Pricing granularity in GCP is per second (with 1min minimum) and you can fire up firecracker VMs within milliseconds as another commenter pointed out.
Cloudflare Sandbox have less functionality at a significantly increased price. The tradeoff is simplicity because they are more focused for a specific use case for which they don't need additional configuration or tooling. The downside is that they can't do everything a proper VM can do.
It's a fair tradeoff but I argue the price difference is very much out of balance. But then again it seems to be a feature primarily going after AI companies and there is infinite VC money to burn at the moment.
I coud easily spin-up a firecracker VM on-demand and put it behind an API. It boots up in under 200 milliseconds. and I get to control it however I wish to. And also, all costs are under my control.
I compared the costs with instances purchased from Hetzner or Contabo here: https://news.ycombinator.com/item?id=45613653
Bottomline: by doing this small stuff myself, I can save 35 times more.
For guide, just follow their official docs. I did those again today, literally copy-pasted shell commands one after the other, and voila.. had firecracker vm running and booting a full-fledge ubuntu vm.
It was sooo damn fast that when it started, at that moment I thought that my terminal had crashed because it's prompt changed. But nop. It was just that fast that even while literally looking at it I was not able to catch when it actually did boot-up.
By the way, two open-source projects already exist:
1. NodeJS: https://github.com/apocas/firecrackerode
2. Python: https://github.com/Okeso/python-firecracker