My target price for HotS is $20, but I may raise it to $30 if I see sufficient evidence that they "fixed" Zerg in the expansion. I have no doubts there'll be plenty of new content, but Steam sales, a massive backlog, and a general disappointment in the changes made to Zerg in SC2 have ensured I won't spend full price (and will probably not spend more than 50% of the launch price) on HotS.
That being said, I hope that the expansion will bring many new players to ToJ and rally those who played when SC2 released but stopped playing in the months that followed.