That's a half mil for one game... agreed. So let's say for two a year, that would be an extra million.
However, the cost of the double deck as said by clones33 before would be close to 30 million. It would take 30 years to pay that off.
So lets assume that in 10 years, the cost to do it is 60 mil (I dont think that's too unreasonable). Instead of looking at a 60 million dollar upgrade, you're looking at paying off 20 million. By having the vision to do it early, you saved 40 million dollars... and that assumes you didn't sell a single ticket for those seats beyond the 2 games per year for 10 years.