Yikes, just saw $20 per 6 week tournament period to participate in the global arena. So to compete at WoW arena will cost you a nice sum of $15 * 12 + $20 * 52 / 6 = $353 per annum.
That is lot of money to spend just to be allowed compete. Unless they plan on removing the requirement to have a normal account and just allow people to only be able to play on arena realms that is going to discourage a lot of players.