I think you have it backwards. If anything we are the most valuable in terms of worth and assets, but the yankees are probably more profitable in terms of merchandise sales, tickets (remember there are a LOT more baseball games than football ones), etc. I would like to see some numbers on profitability. But we are without a doubt the most valuable sports franchise in all of the world, at least according the latest Forbes magazine numbers.
Redskins - $1.423 bill according to Aug 2006 numbers which i am sure has increased since those numbers came out
ESPN - Forbes: Five NFL franchises worth over $1 billion - NFL
Manchester U (2007) - $1.453 bill
#1 Manchester United - Forbes.com
New York Yankees (2007) - $1.2 bill
#1 New York Yankees - Forbes.com
Real Madrid (2007) - $1.036 bill
#2 Real Madrid - Forbes.com