Did you already check out Frappes Cloud? It’s also by the makers of ERPnext but without support. So far I have been very content with it.
Here is the pricing structure: Frappe Cloud Pricing
(Nice addon: you are supporting the maintainers of the project and thus the project itself instead of a random hosting provider)
The pricing is based on data usage instead of per user. I guess sounds more appropriate for your use case. You can just start with the lowest tier and upgrade your way up regarding of the computational need you will have.