go to foundry, there is a questline there. after the questline is finished you get access to top end crafting materials that every single build in the game can take advantage of. the catch is each piece will set you back 3000 stygians, and you will most likely need more than one set.
another money sink is furnishing your home in core city after a megacorp questline but its mostly decorative.
jetskis also cost a lot, with the most expensive one being 25,000 stygians if i recall, and collecting every single one will use up to 100k if you dont have anything invested into mercantile. just keep stacking the cash bro its fun to look at big numbers.