First buy the potent lyrium potion recipe at Wonders of Thedas in the Capitol, then buy lyrium dust at Circletower.

Then buy flasks, concentrator component and distillition fluid at Party Camp.

Use a character with master herbalism to create the potion and sell it at Gorim Stall in market district.

You get 99 potions for 245 gold every time.

Patience is the key here because you will have to repeat the process until you decide you have enough gold.