Okay, so I went through the cards of Karazhan and Gadgetzan and, based on my limited knowedge of the meta, tried to figure out how many cards from each were actually being played in high tier decks during the time they were the most recent release. The numbers I came up with were:
26 of 45 cards for Karazhan
53 of 132 cards for Gadgetzan
Now, keep in mind that Karazhan was considered a low impact adventure, and Gadgetzan was considered to have more useful cards than most expansions. So that kind of skews things, but I didn't realize just how big a chunk of Gadgetzan was actually played. Especially the 11 of 20 legendary cards that saw serious play at some point, at least as far as I know. So it is fair to say that expansions have a bigger impact on the meta than adventures, in general, though I'm betting that BRM had a bigger impact than TGT, so it's not always true.
So, those 26 cards that I counted as highly playable, 14 common, 7 Rare, 1 Epic and 4 Legendary. That's 8,060 dust to craft, but let's assume you'll get all of the commons from packs, half the rares, ignore the epic, and get lucky and get one Legendary in 2800 gold worth of packs. That's still around 5,000 dust needed to craft what you'd get from the adventure. That's a huge amount of guaranteed value from an adventure.
And I stick by my earlier statement. Having a lot of people with limited card collections having to make difficult decisions about what cards to craft (and keep in mind that even people who aren't F2P and bought a $50 pre-order are still not going to have all of the cards in an expansion), only leads to a low-diversity and stale meta where players flock to the most popular and powerful decks because they don't want to waste resources on cards that might turn out to be worthless.