Interesting to compare with Fly's sprites: https://sprites.dev/#billing
"30 hours of wake time per month (~5 concurrent users avg), averaging 10% of 2 CPUs and 1 GB RAM"
Does that mean it would sit available but using 0% when there's nobody on the site, and just bill for usage when web traffic is causing the server to do work? So if the web app went a month with no visitors it would cost nothing (except for the file storage fees)?
Yes that's the idea. The public URL for a sprite is served by a (free) load balancer. The sprite is normally suspended, gets resumed when a request comes in, then suspended again. Not sure on the exact timeouts, they probably don't suspend immediately after a response is sent.
Sprites pricing is based on usage, not reserved capacity, so depending on what you're doing I think it can actually be cheaper than Shellbox. You'll have to stay below 1GB of memory and have the CPU be mostly idle, which I'm not sure common workloads will.