Kagi used to have limits on all plans, and I feel like associating cost to typos is a bad experience that you'd never have on ads-supported engines. Even now on starter plan (300/month) a mistyped query would cost you $0.0166 each.
Now I use the unlimited plan and so I search first, spellcheck later. Or sometimes it corrects it for me.
This only works at the extremes of volume. If you're targetting very-low use users, or enterprise, you can price per search. In between the frictions just don't make sense for any sensible target market.
A sliding scale would make sense. Don't use it? Don't pay anything. Fewer than x searches? Pay Y per search. More than z searches? Pay z * y for unlimited use.
I suspect this would also work great for streaming services, like HBO or Netflix. Rather than paying for unlimited use on 6 platforms OR spend fortunes on pay-per-view on yet another platform, just reward your most loyal customers but keep the door open for incidental users.
Their search API is in beta right now.You can apply for access or wait for it to be released. I guess then making a front end to call it is simple eg an llm could make it.
I put money into the account, you bill me per search - pre-paid usage based billing is the only way this can ever be "fair".