Seth Godin, who I first met in 94 or 95 at an AOL conference, is considered the foremost expert on permission marketing.
I say that the right metric isn’t how much it costs to send a mail. It’s how much sending a mail is worth! In other words, if an industry-wide .25 cent stamp eliminates spam, it’s likely to double or triple the response rate to permission mail. A boon! If you send me 100 emails with stamps, and I read them all, you’ve spent a quarter. If you can’t cost-justify that, you shouldn’t be writing to me. If I were an email permission marketer, I’d love this… the same way the DMA should have embraced the do not call list.