The cost would be minimal. But spammers would face a different hurdle. There would be a real cost to sending their junk. At a minimum, they would have to reduce the inclination to send rubbish repeatedly to the millions of e-mail addresses they collect, whether they are deliverable or not. Such a scenario is a long ways off.
Consumers scream when they lose something free, even in the face of benefits.
But some day we will reach the point where spam constitutes nearly all e-mail traffic. It will create such a burden on the network that legitimate e-mail will be lost or delayed. Then those few pennies will be a small price to reduce the flow of garbage into our e-mail boxes. No spam, please.