While still training (before the training center is destroyed), you will notice that you will receive money (allowance, paycheck, bribe, etc.) 15 C-bills at a time. Just 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. Each location varies by version number, does not work when paused, and will not work after the training center is attacked.