Lets look at it another way - do a google search for star wars and then facebook and notice the number of results:
star wars: About 420,000,000 results (0.21 seconds)
facebook: About 20,570,000,000 results (0.17 seconds)
Now if you get (100/fb results)* star wars results you get a value of around 2
So with that you could say if facebook was overvalued at twice its value then you can say that lucasfilm is worth 4bn, or at least argue that as a price.
Personaly I would of thought that had Lucasfilm ran a share sale, then the value would of been alot more, just from fans collecting a share certificate for there star wars collection alone. Now they can own a Disney share and feel all dirty I suppose somehow.
Either way when you sell a company directly, I tend to see low ball prices in consumer valuations of the company. Now if you sell it as shares or are flavour of the month and can get silly offers, the prices seems to be over consumer valuations. Though even after this news, the share price has not changed much and still values Disney at around 90bn.