-When training (before the training center is destroyed), you will notice that you will receive money (bribe, allowance, paycheck, etc.) 15 C-bills at a time. Simply select a spot in the training center and wait for about an hour. You should have at least 500 C-bills. Note that this only works on some locations on the map. Every location varies by version number, does not work when paused, and will not work after the training center is attacked.
-If you have problems finding that magical spot in the training center, there is another way to getting plenty of cash. First, after each training mission, hang around until the money stops appearing. Then, put it into the ComStar stocks of Defiance Industries, better known as DefHes. This stock will steadily increase as time progresses. If you are careful, you should be able to buy tech training in the MechitLube, all the weapons courses in the Citadel, a flak suit and a mech weapon of your choice. Then, leave whatever money you have in the stock. Should you make it to the Spaceport, hide behind the ComStar building for a couple hours, and presto, instant cash. It may take a little longer, but it is well worth it. Try to leave the game running overnight to get up to 1.4 Million C-Bills.