I don’t think the reward structure is scaled very well between the two. Nightmare being exponentially more difficult, and impossible to complete I think needs to have restructuring of the reward system so that they reflect competing at a much higher difficulty. To me they seem about the same value as regular for doing as many stages.
This comparison is assuming you do as many levels in nightmare mode as you would for regular mode, so about Midway through gold.
Total rewards for regular, completion and Rank 16-50:
9750 league tokens, 8 trainers, 4 legendary pieces of gear, 2 4* weapon pulls, 1500 WD tokens, 100 gold ascendance medals
8000 league tokens, 6 trainers, 12 pieces of legendary gear, 4 gold mods, 2000 wd tokens