Each arena entry costs $1.99 or 150 gold. With each arena entry you get one pack of "expert" cards. That is 5 cards. The same pack costs $1.50, or some small discount with mass purchases. You also get four random rewards that scale to your performance with arena entry. So you are essentially paying $0.50 cents per arena run, plus a mandatory pack purchase. You typically earn enough gold for another admission at 7 wins. Less than 10% of the player pool can win consistently enough in arena to continually fund further admissions through only arena winnings.
The arena seems to be how they intend to make money, as very few people value digital cards enough to outright buy packs. That means they probably aren't going to change the system to something fair like after 3 wins you can fund another run. However, if you do the daily quest you can typically earn an arena run through rewards every other day.
Overall, the game looks pretty balanced now. There isn't that much card diversity yet, which keeps things simple. They might have overnerfed Rogue with the last update very slightly.
