1: It depends on what decks you want to run. There's at least one top tier card per wing (and quite a few in the second wing). Although they are pretty control orientated. Which is generally so expensive that if you were going to play it dropping the cash or the gold for the place wouldn't really be a question for you anyway. If you wanta run shaman though tunnel trog is really good. And in addition many of the cards are playable even if they are not top tier, and in the lower ranks you can do pretty good with many of them. So, I think, generally, at least the first two wings are worth it for your gold. For your cash that's a lot more subjective, I wouldn't do it, as I feel the value in fun it gives really isn't much compared to spending that much money on other games. But eh, whatever you wanta do.
2: Hard to say. There's been times where they dropped the news a month in advance. And others where we've only gotten a week.
3: I have no idea how common they are. I don't use it, but that's about the only statistic I have available. They are very very common among professional players though, so I guess they are not that shady, since blizzard probably would have done something if all the high visibility players are using them (and they are) and blizzard didn't like it.
Edit: To give a better breakdown on the wings.
Wing one has: Reno Jackson and Jeweled scarab, I'd say nether one is really top tier (whoops, sorta shoots my earlier advice in the foot), but they are generally playable.
Wing two has: Tunnel trog (top tier in shaman) tomb spider (dang good in hunter) unearthed raptor (on the edge of top tier in rogue, but a it can be a pretty expensive deck) fierce monkey (playable in some warrior decks) Excavated Evil (top tier in priest, but once again pretty expensive) and bran bronzebeard (top tier in a few decks, notably C'thun decks. Probably playable in other decks.)
Wing three has: Huge Toad (good in hunter) Sir Finley Mrrgglton (good in shaman sometimes, fallen out of flavor I feel though) and also Anyfin can happen. Which I think was sooooo close to making a whole new deck type (combo paladin) until they ripped out my heart and stomped it to the dirt by removing old murk eye from standard. Probably not playable, but sometimes get experimented with, possibly we'll see it's use someday.
Wing four has: raven idol (top tier in druid expensive deck) Museum Curator (top tier in priest expensive deck) Desert Camel (playable in some hunter decks) Arch-Thief Rafaam (uhhhh. I'm not convinced by this card but I've seen it played rarely in some very expensive decks so maybe...) and Elise Starseeker (who is top tier in a couple of ridiculously expensive control decks.)