Upcoming Marvel sets:
Marvel: Greatest Battles (04/17/2013)
Women of Marvel: Series 2 (November 2013)
Iron Man 3
Marvel Legendary: Dark City
Marvel Fleer Retro Trading Card Set (name tbd)
So, that's 7 more Marvel based sets coming out, and there's no guarantee that will be all of them. Even the most hardcore collectors must be feeling some fatigue, and the financial aspect must be forcing even those with deep pockets to be more selective, which leads to less competition for most individual cards, which leads to lower overall prices.