I don't think the current firehose model of spam would be sustainable anymore, though. Those spammers send millions of mails a day. Even with a 1 cent cost, they'd have to be much more selective about their address lists, given the low success rate. It may not solve the problem but I'm almost sure it would help a little. It also may be an additional qualitative barrier for crime-linked spam such as phishing mails, because they'd have to try and find a non-traceable was of payment, which is not trivial and always carries a slight risk of being identified anyway.