Wednesday matinees tend to be cheaper, but not by a very significant amount (for example, tickets that may be $100 at any other performances would be $90 at a Wednesday matinee, etc.) I think it has more to do with the time and the fact that many bridge-and-tunnel retirees find that it's the most convenient performance to attend.
Who are you calling old? You little whippersnapper!
"All I ask of you is one thing: please don't be cynical. I hate cynicism -- it's my least favorite quality and it doesn't lead anywhere. Nobody in life gets exactly what they thought they were going to get. But if you work really hard and you're kind, amazing things will happen."
Conan O'Brien