Hearthpwn

So, the http://www.arenamastery.com/sitewide.php page is cool. I took their information , and started a spreadsheet to work out the answer to this question.

Taking into account the opportunity cost of not getting the 10g/3 wins, the value of a pack (100g) and the average rewards reported by arenamastery, the answer is between 3 and 4, depending on how you value the cards and the dust. My estimates for those things had the answer at 4, but if you put more generous values on them, an argument could be made for 3 wins.

Taking out the nebulous values I had for dust and card rewards, and leaving only the gold and pack as the reward, the answer was a solid 4.

So either way, 0-2 wins is a straight loss, and you were better off just spending 100g on the pack. And 4 wins is a clear profit, and you're better off in the arena. 3 wins is unclear, and depends on how much you value cards and dust.