I'm currently level 10, managed to get the saving box with Kitty and got Rogue to mind-control Xavier into giving me more money, erasing Jean and not bothering us during sexy time.The lockbox money is a one-time thing iirc. Early game the best way may be to just study and go to classes to raise your stipend. If you enact a girl's plan when xavier catches you in public, you can raise your earnings up to a point.
Is that it? is it Micro-Transactions from now on to increase money?