I noticed my sun and moon dragons were producing less Dragoncash than what I remember. I verified the numbers on this site.
I performed a test on my four Double Rainbows. See below.
1 DRD: 500 dc/min
2 DRD: 500 dc/min
3 DRD: 500 dc/min
4 DRD: 499 dc/min
I believe this shows a rule of diminishing returns for Dragoncash, decreasing the value earned per minute by each dragon. The effect seems to increase per same type dragon, but may be per same element type or per number of dragons per habitat type. I will do more research on my sun and moon habitats.