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.
There is no practical limit to the amount of armor you may possess, because while armor can increase without bound on Warriors (and to a lesser extent, Druids), the game will never last long enough to get anywhere close to any programmatic limit (due to Fatigue, which cumulatively increases each time it hits you).
According to the wiki, even if you survive 49 turns over fatigue (by which time you would have been out of cards for at least ~22 turns), the game ends in a draw on the 50th turn.
So Armor Up, and sleep soundly knowing that more armor is never wasted!
Best Answer
There are currently three cards that have an effect on the cost of a hero power, all from the The Grand Tournament set.
These cards are:
Maiden of the Lake: The card mentioned in your original post. It reads "Your hero power costs (1)." This effect lasts for as long as Maiden of the Lake is on the board.
Fencing Coach: This is the card most similar to what you're asking. It reads "Battlecry: The next time you use your hero power, it costs (2) less." This Battlecry effects the Hero Power itself, so is lost if you change hero powers.
Saboteur: Least relevant, but included for completeness. This card reads "Battlecry: Your opponent's hero power costs (5) more next turn."
In conclusion, there is no way to permanently reduce hero power cost below 1.
As of the Mean Streets of Gadgetzan expansion, a new Priest card called Raza the Chained will reduce your hero power cost to 0 for the rest of the match when the battlecry is activated.
While this card is in priest, it has some interesting possible synergies when comboed with Sir Finley Mrrgglton and Kabal Courior, as you could discover a Coldarra Drake for infinite hero powers with the power you receive from Finley.