Using zoo at rank 19-20, the average win time is 360 seconds, which means it would take 10800 seconds to win 30 games. Add that with the time it takes to concede and the possiblity of losing you get around 3 hours and 2 minutes for 100 gold, which is just one friggin pack.
The moment you purchase an arena run you are already refunded 100 gold in the form of a card pack. You just need to earn over 50 gold to earn money because you pay 150 to enter. Reaching 7 wins will probably take 65 minutes factoring in 1 lose. At minimum, you earned 100 gold in the form of the card pack and you are refunded your 150. This means 100 gold every hour, which is way more than play mode farming.
So, how well does the bot fare in arena, default ai and silver fish? We really should be dedicating our resources to arena. Besides, with more people botting in arena the game is pretty luck-based, which allows the half of us to win more.