If you have less then 50 gold(T) kill some enemy at night in the slum (if you have fight skill it will be more easy).
Casino - black jack you can earn easy money but don't be greedy or you will be forbidden to play for a week(game's time).
If you have reached the point where you have your office and desk(North Center) the daily quests are good source for money(and more)
Also invest in your business.
Once a day you can teach students in room 2 /school/ that give you 60 gold a day /you can't teach on the weekend/