Never mind that if you take the $25 at the door price of Worldcon in 1974 and run it against an inflation calculator you get $101.21 in 2004. So that means that the $200 at the door price today is more or less double what it was in 1974. So if inflation is only that much, why $200? Well, my understanding is that certain weekends, like Labor Day, were much deader for hotels in decades past. This meant that conventions could get really good bargains when going for them, but that's not the case anymore. Now we compete with all the other things that hotels have found will fill those spaces.
Another thing is size. In 1974 the Long List has 3,587 as the total membership. In 2004 the equivalent number is 7,094. This means using larger and more expensive facilities, Sheraton Park Hotel in 1974 and Hynes Convention Center, Sheraton Boston Hotel, & Boston Marriott Copley Place in 2004.
And I've heard tell that Worldcons do somewhat more now than in 1974. Fancy shmancy lighting for the Hugos, professional sound systems, projectors for presentations, etc, etc.
So could Worldcon be less expensive? Almost undoubtedly, if people were willing to make cuts. All those little things that have crept in but aren't necessary to whatever a committee's vision of the core mission of Worldcon. But it would still have to be in big convention centers unless they caped the membership (oh, hey, I guess that would keep out the riff raff as well) or settled on a few very large hotels when in the US. Ultimately it is up to the people who vote on such things and they mostly go to Worldcon, unlike the people who whine about how expensive it is. I'm willing to bet even if prices were lowered these people still wouldn't go because they'd be complaining about the hotel, airfare, etc.