This added cost makes spamming more expensive and less profitable, thereby reducing the amount of spam.
That part right there... Spammers already pay for captcha bypassing, so your system will only make their life easier and probably less expensive, because if you wish to make it more expensive for spammers, it will be too expensive for valid users.
But anyway, good luck with your project. I'm curious to see the result and if it will work.
Thanks for the feedback.
I agree with you that the price for getting into a website has to be more than the price it takes for a spammer to get a CAPTCHA solved otherwise we're just making things easier for them. One way to deal with that would be to allow feedback from websites. If after a certain number of days you aren't reported as a spammer, you get a refund of your tokens and can use them again. Whereas spammers would be forced to buy more tokens. That would allow the price to remain high enough to cripple spammers but not inconvenience valid users as much. Also, it would put spammers' resources into Bitcoin mining, strengthening the network even more, instead of putting it towards having low paid workers solve A/V CAPTCHA's. We could also have reputations tied to tokens as well. As you spend tokens and aren't reported as a spammer, you aren't charged as much for new tokens. If you suddenly start spamming, the price goes back up to normal.
