The real money prices scale with the number of wings you buy. Buying one wing is $6.99, but buying four is only $19.99.
By contrast, you can buy any number of packs you want for 100 gold each. If you buy them for cash, two packs costs $2.99, seven packs for $9.99, fifteen packs for $19.99 or forty packs for $49.99.
So buying the expansion for gold costs the equivalent of 28 packs. Buying it for money costs the equivalent of 15 packs. Clearly if you have a plan to spent $20 on the game, you are better off spending it on the expansion and spending the gold on packs. Even if you bought the wings one at a time for money, that would still mean you were buying 28 packs of content for $28. The most efficient price of packs, buying 40 at a time, they still cost $1.25.
I've been trying to get my head around these numbers. My assumption is by making the expansion relatively expensive in gold is because they feel a need to put in a gold sink.
Hearthstone was released on March 11, 2014, though people have been playing for longer than that. But suppose a person played regularly since March. Let's imagine a person who has done a quest every day since then. Even if all quests were 40 gold quests, and the person won only two games per day to complete those quests, that would still be nearly 5700 gold. That's won't buy you enough packs to get all the cards, but that's not an extreme level of play.
It is completely feasible to play Hearthstone without ever giving Blizzard a penny. You'll have to invest several months of play to get to the point that you can build a high end deck, but you can certainly do it. And if you are good you can probably stay afloat playing arena with the help of quests.
Hearthstone streamers who play for hours every day have a lot of gold. From what I understand Massan have never payed for cards and Hafu has 15,000 gold in the bank. I believe at least one streamer has played to legendary in one season using only gold they earned in that season, though I may have misunderstood.
Any amount of gold that a typically player can afford will be free to people who play obsessively. Without a similar gold sink every few months there will be no reason for existing players to keep paying in. For the rest of us I guess they are trying to make the numbers loudly say, "Pay us money for this... money!"
Not that I think Hearthstone is doing badly. Apparently analysts think it will make between $100M and $200M this year, and to be honest I wouldn't be shocked if it was higher. That analysis might have been done before they announced that they were going to be asking their 10 million players for $20 each for the expansion.
Looking at the pricing, it makes me want to buy the expansion for cash and use my gold on packs. Now this has a lot to do with me wanting to open a large number of packs to get data on rarity distributions since I can't find it online, but if they can set a pricing structure that makes me instinctually want to give them money, then I think they have probably set the price right.