The revenue per minute is calculated as an average. In short periods of time (minutes), it may not be totally correct as each play area requires more or less time to be played. But in long periods of time (hours), the income per minute will be correct.
The total revenue of a dungeon is the lowest value between the Crystal Converter, the Wisps and the revenue of all areas, therefore it is important to keep all of them in balance. You can evaluate these numbers in the statistics screen, that can be found in the top left of your screen.