The revenue per minute is calculated as an average. In short periods of time (seconds/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 your casino is the sum of the revenue of the entrance, the parking, and all play areas. You can evaluate this number in the statistics screen, that can be found in the top right of your screen.