Presumably because they want to be able to justify the pricing by telling you all about the new stuff they put into the thing, and they want to time details of the product with a big unveiling and marketing push.
Just announcing the price without details of what you're getting for it is just going to make a bunch of forums and subreddits very angry in the way only gaming communities can get really angry.
If they're shipping by Q1, then they know exactly what the details / specs are.
If they haven't released the price, it's more likely because FB wants to throw some serious money behind the marketing push. Which either means they were going to anyway, or the price is going to shock people.
Sure, they definitely know what the details and specs are, but they also want to make a big deal out of said details/specs, and dropping it silently on a web page isn't anyone's idea of a big launch.
It's likely that there are substantial differences between the product and the DK2, or even what's been discussed in previous interviews/previews, so it makes sense to not talk too much about the product until they're in a position to present it properly (e.g., a big keynote followed by journalist demos).
And yeah, it's going to be expensive - the fact that they've said as much in previous interviews means they've already been trying to soften the ground for the revelation. It's almost certainly going to be more expensive than the DK1/DK2.
Standard marketing BS these days [0]. Announce the coming announcement for the pre-ordering of an unreleased product. A fake news event to generate the exact press they're getting.
I wouldn't be surprised if they announce the price before that timer goes away, just to get another round of press.
This announcement is just a courtesy heads-up to people who are waiting so they can arrange their schedules. The actual announcement is Wednesday. They didn't announce the price because then people would write news articles about it and they want to be able to do their marketing pitch at the same time and convert the buzz into preorders.
Just announcing the price without details of what you're getting for it is just going to make a bunch of forums and subreddits very angry in the way only gaming communities can get really angry.