There should be a coupon enclosed with the first adventure; although I don't know if it still works now that RPGNow has been redesigned.
The $34.99 was an introductory sale offer (it was only supposed to be available to those who pre-purchased the subscription before we'd even released the first adventure, but for some reason ended up being there for longer than we wanted); at that price we don't actually make any money at all - the reduction was our profit. The idea was to get people talking about WotBs before the launch in the hope of generating sales later.
In addition to that, RPGNow increased its commission, which meant that, instead of merely not making any money at $34.99, we were actually losing money! At present, the $49.99 for twelve adventures (minus the $6 coupon in the first adventure) is the rock bottom price we can do it for and afford to make any more adventures in the series. It works out to about $3.50 per adventure (some of which are over 100 pages long), which we honestly feel represents incredibly good value. We hope you guys agree, but we really can't afford to hand 'em out for less than $3.50 per adventure!