Based on some completely suspect math, having 1000 engineers available for the length of the conference "costs" apple ~40 million dollars in productivity:
I'd be almost willing to bet that because the cost is so insignificant to them, the WWDC ticket price is almost entirely meant to set a bar for the types of developers that Apple wants to attend the conference. Specifically, professional developers who make money from the Apple ecosystem, to which the ticket price is a pretty insignificant expenditure for the hopeful value that they recieve from being there.
I hear they made a rule internally that WWDC is supposed to pay it's own way. I certainly agree that it doesn't need to, but just thought you might find that interesting.