The problem with a one-time fee is the server costs are recurring, not one time (though generally I agree with you).
Discussion
Is a backend required for nostrgram? Probably we can offload the static parts to static file hosting like Cloudflare pages to reduce some costs.
Yes it uses a caching server and a web server so cannot be offloaded (though I already use CloudFlare which helps a lot with the web server load via compression and caching).
Yes, I meant more in general terms. Have you thought about having open books? Just spitballing here, but I have no idea what your costs of operating would be. I’m assuming many wouldn’t either. Maybe people would contribute more, if they knew the costs? Like a co-op kind of thing?