The new HD content is sold at a premium price (typically $20), and as far as we can tell that pricing stands regardless of whether or not the movies offer iTunes Extras. A few titles, mostly items that were already available on iTunes, sell for less -- such as Terminator 2 [iTunes link] for just $13.
Until now, Apple has offered some of its HD content as exclusive Apple TV rentals. Items could be rented from Apple TV units that were not available to rent from the normal iTunes Store. It's unclear at this time whether Apple will continue this policy of exclusive Apple TV rentals, or will expand the iTunes store-based rental system to match the Apple TV offerings.