Hearthpwn

The answer is - it depends. There are different ways to play hearthstone and that means there are very different prices. Price is different for

1) Playing casually with whatever decks you can assemble
2) Having 1 meta deck 
3) Having several meta decks
4) Having all standard cards
5) playing only battlegrounds
6) and so on

Some are very expensive, some are very cheap. For example, it is trivial to get 2,5K gold per season as battlegrounds-only player, especially if you are willing to spend some time doing non-battleground quests.