Spam now equates to 92% of all email. Spam grows roughly 33% each month that means Spam increases by over 117 billion emails every day.
I have to think that the response rate is very low, but spam is very cheap. This is to a greate extent because spammers often make unauthorized use of third party computers and address lists. The come ons are frequently for fraudulent offers as well.
For now, junk mail filters seem to be the primary means by which the public controls the signal to noise volume. But, at some point the burden of keeping spam under control is going to reach the point where some better policed alternative to the Internet (or subset of it, like approved sender only spam filters with a third party sceening step run mostly by human beings) are going to kill conventional email.