In hard copy, $5 each. So $30 plus postage. Not sure if it's a sale or if that's the long-term price going forward.
Second Darkness has a negative reputation as one of the most problematic APs. That said, there's probably $30 worth of good stuff there. The first two modules are generally considered decent-to-good. There are some worthwhile encounters and NPCs s in the later modules, along with some interesting articles and whatnot. And if you're the kind of DM who views APs as raw material that you're going to rework massively anyway, then hey, you could have fun with this.
Doug M.