L38+ would be plasma pyramid soloing ( dont fight the bosses or mini-bosses) makes around 2k gold per run with gold and liqudating
L63+ would be Humania sandbar lagoon or devils coast, makes around 1.2k gold per run with bosses.

An Elixir would have a chance to boost the average gold per run...
In my perspective:
Humania with elixir= 2k per run ( sell the purples? )
Plasma pyramid= 5k per run ( with bosses )