Forbes' list of the world's most valuable sports teams shows you the power of the NFL, as the top 50 is littered with American football franchises (I specify American football because quite a few soccer teams are on there as well). Even the Lions, which haven't had any success in the last decade and are in a struggling economic market, came in as the 36th most valuable team in the entire world.
Forbes values the Lions at $867 million and lists their revenue as being $206 million (imagine how much they would be worth if they had even a little success on the football field). Based on those numbers, the Lions are the 28th most valuable franchise in the NFL (ahead of the Jaguars, Falcons, Vikings and Raiders). Like I said, this list shows you just how big the NFL is. $867 million seems like a lot of money, but that's only good enough for 28th in the entire NFL.
The Lions were the only Detroit team on the list, which ranked the top 50 most valuable teams (every NFL teams was on there, by the way).