I stumbled into a pile of C-Bills yesterday, and here's how:
Doing the new tutorial pays pretty well. I did part of it and got 2.5 mil
Played 1 round of Community Warfare under the new FRR contract, decent winning round paid 400k, plus 150k loyalty bonus, plus another 400k and 3000GXP for ranking up under FRR.
If you have any of those medallions from pack purchases ( I got 4 from Phoenix ), cash them in for FRR and equip them in the mechs in your drop deck. Extra loyalty points give extra cash bonus for wins.
So, about 3.5 million C Bills for an hour's work. Even if you can't stand playing CW, grind through that tutorial for about 15-20 mins and come out 2.5M richer.