If demand is down, I assume ticket prices, concession prices, and parking prices will go down. Right economics majors?
As for concessions, once the fans are at the game, they continue to be a captive market, and have only one place to buy concessions, so demand will continue to be inelastic, just at a lower level. Costs for the stand are relatively fixed, and costs need to be covered, so prices may actually rise as costs are spread over smaller volume. What you probably would see happen is that, if attendance falls, there would be fewer stands. As an example of this, at the EMU game that I attended last year, there was only one concession stand open at all on the Ohio side, and it closed at the half.
As for tickets and parking prices, yes, falling attendance should lead to lower prices for those. Since they don't like to reduce the face amount, you would probably see more promotions.