Hearthstone card packs are 100 gold per pack. An entrance to the arena costs 150 gold. Have these prices ever temporarily been reduced for a sale or special? Has there been any mention of sales by Blizzard?
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.