Timothy Taylor thinks we may have reached it:
[I]n 2012, it looks as if the tide may be turning against e-mail spam, at least a bit. Some evidence comes from monitoring of spam done by Kaspersky Lab, a seller of information technology security services. In particular, Darya Kudkova has written the “Kaspersky Security Bulletin: Spam Evolution 2012.” The [above] bar chart, from the Economist magazine, [shows] Kaspersky Lab data on monthly patterns of spam from 2006 up through 2012.
[T]he economics of sending and screening emails, together with the economics of online advertising, is tipping the balance a bit for spammers. Getting people to click on offers in random emails is becoming more costly; getting people to click on random advertisements is becoming easier.
Previous Dish on the cost of spam here.