It's more like "Purchase _Halls of the Mountain King_ and have the option of purchasing _Wrath_ at some point over the course of the patronage."
_Halls_ is projected to ship in...June,
iirc? So you'd need to scrounge up about $5.50/month to pick up _Wrath_ before the option disappeared.
In which case, you end up with two adventures for
4E, one of which you had the opportunity to contribute to the development, and about six months worth of interaction and content...
For me, that's not overpriced.
-Ben.