It's the end of summer, and that means that businesses around the country are being emptied of their interns. Early this summer, they arrived with their youth and their ambition. As the air turns crisp, they go home, leaving behind neatly stacked piles of folders, well organized databases, and, perhaps, a good impression.  

But a small group of those interns left something else behind: a wad of cash. Today, a growing number of young people – or their parents – are paying thousands of dollars for the privilege of working an unpaid internship.