I already knew that posting several comments one after another increasingly makes it more expensive. I just noticed the same thing posting 2 posts in my ~science territory. The standard fee is posting fee is 100 sats there, my second post cost 1000 sats (I clicked post 1000 sats without noticing). A third one would have cost me 10,000 sats, I stopped just in time.

Not complaining, just an observation.

This looks like a pretty good approach to avoid spam.