The National Retail Federation estimates that Americans will spend the following for Halloween in 2011:
Total Halloween expenses—$6.9 billion (that's more than twice the amount Americans spent for Halloween in 2005)
Largest Halloween expense—$2.5 billion for costumes
Amount spent on candy—$2 billion
The research also shows that Halloween isn't just for kids anymore. In 2011 nearly 70 percent of adults plan to hold their own celebrations for Halloween.
Off the top of my head, I can think of at least three factors for this accelerating trend:
Lots of adults wanting to have a good time (less charitably put: another excuse to party and drink).
Aging Boomers not letting go of their youth.
Secular holidays and their celebrations/spending are replacing religious holidays. Christmas is, for all intense and purposes, a secular holiday in the minds of most Americans. Even Easter has been slowly taken over by the Marshmallow Peeps Brigade (Full disclosure: I love marshmallow peeps).