I don't know if it was asked in this thread, but I've seen it asked elsewhere, and my arena deck is in the perfect position to answer it, so I'll put it here for reference:
Q: At what win/loss ratio will decks 'break even' for rating?
A: My deck just happens to have a 14-4 win/loss ratio right now and has exactly 0 rating (in platinum) so at least right now it's roughly a 3.5:1 ratio, but that will probably evolve over time, since the formula for arena rating is unknown. It's 10 points for a win, but losses I think it depends somehow on the opponent who beats you, because I don't think it's a constant number.