Yay, whitelisted!
DDOS are usually made from botnets with thousands of compromised machines, you cannot charge the machine owners or ISP's for that traffic
You see, in a way every packet you send already has a cost (even if you are on what is deceptively called "unmetered")
The idea here is to have a tiny payment to the receiving side being "attached" to every packet, so that every site you visit receives a tiny tiny (under normal, non DDoS situation) payment
The biggest problem with this scheme is that it creates several perverse incentives. For example, sites with no value whatsoever and very low bandwidth could use malware to launch DDoS attacks on themselves to make money. Even if they only had a miniscule amount of bandwidth, they would still get paid for packets that never made it to their pipe.
Well, some folks already more or less do that when they hire botswarms to hit their ads and counters, though the "pay packet" scheme would indeed somewhat increase the incentive to attract even more "non-consensual" traffic.
Also, most sites have saturated outbound bandwidth but plenty of inbound bandwidth to spare. This would given them a perverse incentive to induce visitors to send them useless data as much as possible to fill up their inbound bandwidth to make more money.
Well, aside from crude redirects and botnets, how would they do this ? Cajole folks into uploading random crap ? File hosting services would be very happy, though...
And even if the 'victim' site gets the money, the DDoS can still do damage way above the value of the compensation.
Well, yes, but it is still better to have (damage - compensation) than just damage
Also, with the technology needed to do this, you could implement much better solutions. The problem is largely that the technology doesn't exist, not that there are no good ideas for ways to solve it if you get to assume anything is possible.
Of course, the technology is (almost) completely hypothetical, but methinks it's still a nice idea worthy of some (fairly relaxed) contemplation.