[LeifCarrotson]

I think the most promising price scheme is one where the receiver can set it to arbitrary values. Are you in my contact list? Your price is zero. Are you one of my suppliers that likes to send me "new product" notifications every week? You can still send those, because I need to receive notifications from you, but your marketing department has to really want to talk to me because this will cost them 600 seconds of CPU. Did you come across my email on the Internet, and I don't know you at all? Sure, my email is in my signature for anyone to parse, but when you put it in the "To" field of your email client it will inform you it will take 300 seconds of CPU.