I might be wrong, but I think winning a nightmare difficulty map gives you the same coins as all other difficulties. I think it would be more fair if the amount of coins you get is depended on the difficulty of the round.
So for example. X = amount of players for the map
Easy = 2/3 * X
Normal = 1 * X
Hard = 1 1/3 * X
Nightmare = 1 2/3 * X
Each number is rounded up for when you get decimal results. So a 6 player map will give 4 on Easy, 6 on Normal, 8 on Hard, 10 on Nightmare.
I don’t think this should apply to single player. 1 coin for any win is fine I think and harder difficulties are a nice challenge for yourself to have fun with.