Historically documented to have been purchased by Dutch colonists from Native Americans in 1626 for the equivalent of US$24, Manhattan real estate has since become among the most expensive in the world, with the value of Manhattan Island, including real estate, estimated to exceed US$3 trillion in 2013. On business days, the influx of commuters increases that number to over 3.9 million, or more than 170,000 people per square mile. Many districts and landmarks in Manhattan have become well known to New York City's approximately 50 million annual visitors.