If the transaction cost can become insignificantly low, then money could flow in the direction of the victim. Each attack source could end up making a small contribution to the victim's bandwidth bill.

I'm not sure economic incentives will lead to this happening. There's also the difficulty in attributing traffic to the person who requested it ("is this a request, so we should bill the packet source, or is this a reply, so we should bill the packet destination?").

But in theory, I don't see the DDoS pattern (a high number of attack sources) making any difference to this proposal.