Use the following trick to start story mode with 1,616,320 credits. Start a story mode game and choose Las Vegas as the city you wish to begin at. Note: This trick might work in other cities as well, but is untested. Pass the FIRA exam. Use the "Next Day" option in Hard-Wired to move forward in time and use the following actions to rake in the credits.
Day 004: Go to The Mechanist and buy as much Ventuno Stock (in Option Mods) as you can. It should cost 4,664 credits each.
Day 011: Sell your Ventuno Stock. You now have 269,604 credits.
Day 012: Buy Solid State Stock. It should cost 8,805 credits.
Day 032: Sell Solid State Stock. You now have 468,924.
Day 037: Buy Ventuno Stock. It should cost 6,045 credits.
Day 041: Sell Ventuno Stock. You now have 541,304 credits.
Day 047: Buy Solid State Stock. It should cost 11,932 credits.
Day 058: Sell Solid State Stock. You should have 646,424 credits. Buy Ventuno Stock. It should cost 3,527 credits.
Day 067: Sell Ventuno Stock. You should have 1,135,400 credits. Buy Solid State Stock. It should cost 12,793 credits.
Day 076: Sell Solid State Stock. You should have 1,616,320 credits.
The actual Day you start doing battle in the arena does not seem to affect the game at all. Note: This process can be continued to create as many starting credits as desired. Just like in the real-world stock market, buy low, sell high and you will make money.