Researchers conducted a survey for the National Retail Federation in the United States and found that 53.3 percent of consumers planned to buy a costume for Halloween 2005, spending $38.11 on average (up $10 from the year before). They were also expected to spend $4.96 billion in 2006, up significantly from just $3.3 billion the previous year.  The troubled economy has caused many Americans to cut back on Halloween spending.  In 2009, the National Retail Federation anticipated that American households would decrease Halloween spending by as much as 15% to $56.31. In 2013, Americans spent an estimated $6.9 billion to celebrate Halloween, including a predicted $2.6 billion on costumes (with more spent on adult costumes than for childrens costumes) and $330 million on pet costumes. In 2017 it was estimated that Americans would spend $9.1 billion on Halloween merchandise with $3.4 billion of that being on spend on Halloween costumes.. How many dollars (in billions) has overall Halloween spending increased from 2006 to 2017?