No one talks about new Railway Sandboxes - https://docs.railway.com/sandboxes
I think they have one of the best sandbox environments on the market with pay per utilized resources pricing, it's a huge cost reduction for agentic workloads when you have 95%+ idle CPU time and occasional spikes for CPU heavy work (e.g. agent run tests or something like this).
I use railway to host my openclaw like personal agent for friends and family (9 instances) and it costs like 1-2$/mo with scale to zero.
Have you tried using unikraft? I think it might be cheaper imo. Worth a try.