(Time Entertained)/(Price for Entertainment) = (Entertainment Value)
Simply enter your numbers and literally prove how much more cost effective playing video games is than going to the movies. For example, let's say tickets are $10 (low-balling it) for a 3 hour movie. The equation looks like this:
3/10 = 0.3ev
Now, if we plug in the numbers for a video game with the assumption that you are going to spend at least 30 hours over the course of the game and a standard cost of $60, we get this:
30/60 = 0.5ev
Likewise, with a game like Diablo 3 (or Skyrim, SC2, Angry Birds, etc) where insane amounts of hours can be racked up, we can get something like this:
180/60 = 3.0ev
There you have it, mathematical proof that video games are more cost effective for your family than going to the movies.
p.s. This is in no way an argument for or against the state of D3, just something fun that I try to use when discussing the entertainment value of gaming with my wife. It hardly works but it's still worth trying now and then.