"Relocations and new stadiums are fuelling big increases in the value of National Football League teams, with the Cowboys the economic model every franchise hopes to match", said Kurt Badenhausen, senior editor, Forbes Media.
During the 2015-16 season, Forbes claims that United generated $765 million.
A year earlier, the Broncos were valued at $1.45 billion. Alongside Adidas, the club are sponsored by other big companies such as Chevrolet and 20th Century Fox.
Madrid are valued as the world's fourth-richest sports team at $3.64bn, with their arch-rivals Barcelona not far behind on $3.64bn.
Owner Jerry Jones bought the Cowboys in 1989 for $150 million, including $10 million to cover deferred salaries, according to Forbes, and built America's Team into a $4.2 billion franchise.
Behind them are Manchester City, named as the 35th richest team in the world with a $2.1bn valuation, Arsenal at 43rd with a $1.9bn valuation and Chelsea at 48th with a $1.8bn valuation. That number makes St. Louis the seventh most valuable franchise in Major League Baseball. That's an increase of $300 million dollars over a year ago.