Good question! We also have Uruky Site Search, our own index, which is quite tiny at the moment, but we hope to grow, eventually. Kagi's own index is Teclis, which is one of their smaller providers as well.
I'm only paying Mojeek about $10 - $20 per year in API for my personal metasearch, so I guess this is a terrible market to enter ;) But I'd genuinely be interested, especially if the money is going towards building an index.
I'd recommend you look into any of our other search providers if you just want the API search!