I think the numbers on Trek fans are grossly exaggerated. Original fans of the original show are in their sixties by now, probably not buying a lot of legos. More modern fans probably like other iterations besides the Kirk/Spock version. I dunno, I've always looked at Trek fandom with a healthy does of skepticism. Seems like they desperately want it to be a bigger deal than it actually is. Space franchise envy...
But again, all of the big MB sets like the Enterprise, Technodrome, and CoD Hovercraft had oversupply due to TRU. Everyone bought one at release or on clearance, and supply still exists, thus they're cheap. I anticipate no such problem with Greyskull, as the number of outlets for big-$$$ sets has shrunk substantially. I expect people who wait on the castle will end up being really sad when Amazon sells out, no one else carries any, and they suddenly cost $800 on ebay. Cue nerd-rage.
Based on the parts count, it's about 30% more than the Hovercraft. That's why I think it should be closer to $200-$250, but again, it'll probably sell a fraction of the units, thus requiring a higher cost.