Cool! This is something I've been thinking about and wanted to build too.

I don't want it to be like I'm a valuable person and lower people pay for the privilege of my attention, but I do like the idea of making it so that senders have skin in the game and can't just infinitely generate emails that waste other people's time.

What I'd like to see is different costs based on how I classify the email.

So, everyone except trusted contacts pays $5 per email to me. If I think your email was pure spam, I keep the $5. If I reply, you get your money back. If I do nothing and never classify the email, you get back $4 after 30 days. And I can manually override like reply and keep the money, but those are the defaults.