Manchester United do very well in shirt sales. Topping the Nike shirt sales made in a year, the only club to have sold an equal or higher amount of shirts is Real Madrid, who sell 1.4 million as well. Income made from the Nike shirt sales would be quite high. Since they generate a lot of income for Nike, its hard to imagine that they will not be getting an extension on the contract that allows Nike to manufactor United's shirts
Manchester United's 2012-2013 kit (Home)
Manchester United's 2013-2014 kit (Home)
Manchester United have sold an average of 1.4 million shirts per year over a time of 5 years. This means that even without doing well in the owners attribute, their shirt sales contribute to the total revenue made each year.