I thought you pay gold to enter the arena? The difference there versus real money is that just like most freemium games, you can earn gold in the game. In TB's latest video on the changes, he talked about gold rewards being increased substantially.
From what I've seen of it, you have to pay at least some to play in the Arena. Blizzard has stated they will be giving you all the gold you spend in the beta back after the game goes live.
And the problem with games like this that 'reward' gold is that you have to win to get them. Even MTG, if you won a draft, you could use the packs to enter another draft, however, you still had to pay with a coin. It made it cheaper, but not entirely free. The thing it had going for it was the card marketplace. You could sell your rare cards for currency in the game to be used to pay for more drafts. Plus, you kept every card you got from the drafts.
As far as I can tell, you only get the "rewards" after the arena and it is like a few cards and a bit of gold. Not exactly worth the $3 or whatever it costs to enter.
I guess it wouldn't be so bad if this was a physical trading card game, because you got physical cards. However, the fact that it is only online kind of irks me as just a way to grab money with microtransactions for a largely pay to win genre. If you get nothing substantial from Arena, there is no reason it should cost any money.