I've got a lot of subscribers through betali.st (can't recommend it enough) and it costs me exactly 0. Reddit promoted post generated some subscriptions, but these worked out to about $5 a piece. StumbleUpon cost was +inf, because not a single Speedy Gonzales of theirs managed to stay on the site for longer than 3 seconds, leave alone signing up.
Average time for StumbleUpon visitors on one of my sites is 1.5 minutes, but others only get about 20 seconds of their attention.
I've found visitors from search engines tend to hang around longer, probably because they were actually looking for something. Stumblers and some other social networks are just looking for something to amuse themselves for a few minutes, so tend to be pretty quick to leave.
Interesting write-up though, and I look forward to seeing how other advertising methods perform.
(As an anecdotal sidenote: my work's firewall blocked the bsa.ly ___domain on the basis of "adware".)