Not $100 :)

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.

I posted a bit more details on this earlier, have a look if curious - http://swapped.tumblr.com/post/22485539186/pre-launch-promot...

(edit) And, why is this at -1?