It appears to vary, depending on the Girl Scouts organization selling them. I just removed this small paragraph:

As part of the change in organizational structure in August 2006, this variation was ended, and all troops now sell cookies at the same time, in the early months of each year, and at the same price, $4 per box. http://www.girlscoutsnorcal.org/pages/product_sales/cookie_sale.html

The linked page, http://www.girlscoutsnorcal.org/pages/product_sales/cookie_sale.html doesn't say anything about a national pricing policy, just what the price is from Girl Scouts of Northern California. And the price there is $4.00, whereas, my daughter's scout troop, in Philadelphia, are selling them for $3.50.