Currently travelling the Estonian capital Tallinn, and here they give you the most suitable pricing model automatically.
1 ride = 2 Euros
2 rides = 4 Euros
3 rides = 5,5 Euros (day ticket)
4 rides = 5,5 Euros (day ticket)
This happens as you go - you don't have to decide in advance.
Wouldn't this be an appropriate model for Kagi as well?
0 searches = 5 USD
300 searches = 5 USD
500 searches = 8 USD
1000 searches = 10 USD
At first sight, it seems like you might lose money (users paying "optimal" prices instead of overpaying in a month in which they search more/less as usual). But if you consider image - Kagi as the search engine that treats you fairly and doesn't charge you for what you don't need - this may be priceless. It could also help win a lot of users over who still can't imagine paying for search in the first place.