Wojtek addressed most of your questions, but I'm just going to add details to this one question.
**Are there certain cards that are more cost effective to DE? **
There are two things that affect how much dust you get when you disenchant a card.
- Card rarity
- Foil/non-foil
Card rarity can be found by looking at the gem in the center of the card. Common cards have a white gem and give 5 dust when you disenchant them. The colors match the colors in World of Warcraft, with the highest being orange (legendary). Cards that are more rare will disenchant into more dust (you can see how much as Wojtek explained). Also, the cards you get as you level your hero to 10 are considered "basic" cards and don't have a rarity gem. They can't be disenchanted.
- Normal Gold
- Craft DE Craft DE
Common 40 5 400 50
Rare 100 20 800 100
Epic 400 100 1600 400
Legendary 1600 400 3200 1600
If you go through the math to complete a complete set of 2 copies of every card (leaving all of the disenchanting and crafting until last), you're much more likely to need to disenchant commons and craft legendaries. source
Sometimes you can find gold (foil) versions of cards when you open packs. These cards functionally play exactly the same, but if you disenchant them (and keep the non-gold versions of the card) then you'll get more dust. On the other hand, gold versions are animated (some are QUITE nice) and a nice thing to show off.
So, to answer your question, it is generally more cost effective to disenchant the gold cards, if you don't care about the aesthetics and status of using the gold cards.
Another fact to note, if you use the "disenchant extra cards" button, it will automatically keep 2 regular and 2 gold copies of each card (for a total of 4 copies of the card). You have to manually decide which cards you want to keep and disenchant the others.
What can be won in the arena?
You get up to 5 rewards on completing 12 wins or 3 losses. One reward will be a card pack. The other one to four rewards are random (dust, gold, single cards). More wins = more and better random rewards.
What is the minimum price received when loosing 3 times?
You always get your card pack. The worst random reward is "5 dusts". However, there is MMR matching in arena. You should expect to win at least one game on average even if you're terrible at picking cards.
When accumulating gold through daily's what is the best way spend this gold?
- Dailies in play mode generate 1 pack per ~3 games.
- Arenas generate 1 pack per 5-15 games played.
- Games in Play mode without dailies generate 1 pack per 30 wins (now limited to 1 per day).
There's two extremes of player here: Some players complete the dailies and log off, sometimes going for multiple dailies at once (3 or fewer games per day). On the other extreme, others play at least two hours or more each day (8 or more games per day).
- If you value your time highly (as the first group does), doing the dailies in play mode generates more gold than arenas. Arena purchases wouldn't get used, so packs should be purchased instead. You may want to set aside 300 gold in case you get an extra burst of playtime so you can arena on those days.
- If you value your time playing at a net of zero (as the second group does), doing arenas generates more packs than play mode.
Best Answer
As of now, there has been no such announcement. As far as my memory reaches, the pack price has also not changed.
What has changed, though, is the Arena price. Originally (early closed beta), the Arena was called "The Forge" and required actual packs as an entrance fee instead of gold. In exchange, you would keep all the cards you drafted. And the rewards were also different. But since this was an early beta experiment, it's probably safe to assume they like it better the way it is now and won't change it again (especially since beta is over).
If we look at other big freemium games, we can see that there are frequently sales or special offers, but they usually affect real money prices only. So even if there will be such a sale or offer one day, it probably won't change the gold price, but instead alter the gold-to-money ratio.