Vlad , I really admire the @mphi behavior and proposal.
However, I really don't think that people will use that. At least, thinking about me, if I'll choose to pay on overspending, I think that I'll check my spending at least every 2 days, because I don't have a feel about the number of queries I do, and I'll have a remote thinking like "oh my god, maybe this month I'm spending 200$ and I'm not aware of this". Or maybe I don't have a sense for my daily usage. Or my little search variations (typos, the same search tried with lots of lenses, etc).
I think that the user feeling will be "I'm often anxious about that", and this gives a bad product relationship.
But the @mphi proposal is good for us all, so what could be an alternate proposal? we can think about an approach like "hi, if you'd like to help funding our development and you're an heavy user, you may like to add something to your fee. Last 3 months you've spent an average of YY$/month in searches; how about to add a recurrent tip of Z$/month? Or something else, if you prefer".
In this way, if a loyal user will pay 10$/month, and realize that in the last 3 months he has consumed 18$/month, he could start a recurrent tip of....maybe 3/5 $/ month?
In this way, he'll have a good feeling about kagi transparency, about kagi fairness, about his own generosity, and a correct cost understanding. Kagi, on the other side, will earn less money from the single "exact overspending" user...but I think that more users will tip kagi in this way, so the total gain will be positive for us all.
What do you think @Vlad ?