Is the Monthly Pass worth getting?

Yes, it is worth it especially if you'll be playing the game regularly. The monthly pass will give you 300 BC outright, and you'll get 100 BC for 30 days. The monthly pass costs 30 Rainbow Cards. In the game's Black Card Exchange, the rate is 1 Rainbow Card to 10 Black Cards. So if you convert 30 RC, you only get 300 BC. Since the Monthly Pass already gives you 300 BC instantly, that conversion is already covered. Now, if you signed in the game for 30 days, you'll receive 100 BC daily, for a total of 3000 BC (if you didn't miss a day of signing in). In total, you'll get 3300 BC for the price of 30 RC.

[Reminder] Unfortunately, due to the game's questionable pricing, you can only get 28 RC for $4.99, forcing you to overbuy and get the cheapest recharge. (5 RC for $0.99) Remember that this is just a game so spend responsibly.