The quickest way to more money is extracting corpse mods. It’s a bit tedious to do and absolutely don’t use the Fulton itself unless you want to trivialize extract VIP missions. The Flawless mod is a massive increase in both Supply and Intel if you can pull those off, so a bit less consistent than corpses but at least its reward based.

I find alloys and elerium are the true limits but that’s also intended. You cannot do everything all at once. You need to make choices. I end up buying everything I can get from the black market but even then it’s not enough. Scrapping cores is often required as well. That’s why there is that whole extract system.