The problem with a one-time-fee for a cloud service is that it's unsustainable. That's why I suggest OP have constraints on accounts unless paying a subscription fee. If you really find a 3rd party cloud service (requiring servers/networking/bandwidth) valuable, you should assist in its survival. The fee should be nominal (unless real support is included) but non-zero.

I agree, and that's why, although you maybe thought somewhat outrageous, if OP is going down the 1-time path, i'd rather pay a lot in hope that they can keep the cloud services running for years even if they decided to stop working on it. I like your formula approach.

What canada_dry said below. Especially for what seems to be a solo developer, I'd rather this was costed so as even if they decided to stop working on it, the margin per month was such that I could continue to use it for years. I feel like for a solo developer doing a 1 time fee, more likely to disappear after a couple of years

I second this. Most people I know, including myself, would much rather have one time purchase.