When Breitbart runs an article about declining prices for NFL tickets, they're not talking about NFL teams reducing ticket prices in the face of declining demand. They're talking about the prices of tickets through secondary ticket brokers like StubHub. The NFL teams already sold those tickets months ago, and have pocketed all of the revenue for the full face value of the tickets. It's the ticket brokers who are getting hammered here.
If the NFL is really going to see a decline in ticket sales as a result of this national anthem idiocy, it won't happen until season ticket renewals (and non-renewals) start coming in for next season. I can already see evidence that next year is going to be a disaster for the NFL. One Freeper who was on a long waiting list for the Washington Redskins -- and was expecting to remain on that list for years before he had an opportunity to buy season tickets -- recently began receiving offers from the team for 2018 season ticket packages.
I have seen reports that people way down on the waiting list for season tickets being contacted about being at the top of the list, now and solicited to order their tickets.