This screenshot was taken of
stubhub.com. It shows prices to five different NFL games, all played this past Sunday.
This is a good illustration of how demand for a product affects its price. These stadiums aren't dramatically different in size - all can hold tens of thousands of customers. By December, however, some teams are no longer in playoff contention, while others are. Three teams contending for playoff spots, the Dolphins, Bengals, and Lions, all have minimum ticket prices of about $50 while a fourth, Cleveland, has a minimum price of $31.
The two teams that aren't in playoff contention that are hosting games are the Jaguars and the Redskins. Their minimum ticket prices are $25 and $6.
With a higher demand and a fixed supply, the price should increase to ensure an equilibrium. That appears to be happening in the market for sports tickets.
Another note - given that many tickets are purchased months in advance, this equilibrium can only occur when there are legal markets that allow tickets to be resold.