Hint: Easy money:
Begin a mission and select a territory. Accept the mission brief. On the Team Selection Screen, highlight the Equip command and press A. Buy a Pistol (no cost) for all eight slots on all four agents. Press B to return to the to Equip command. Press B again to enter the inventory box of the current agent. Press A. Instead of selling the weapon, use the cancel command. On the equipment list, press Up and highlight any item except for the Pistol. Press A. Choose the Sell option to get the amount of money that item would cost if purchased, while one pistol is deleted from the agent's inventory. Repeat this process to accumulate a large amount of money.

To get the most cash return from each pistol, select the Gauss Gun as the item on the equipment list. This allows $50,000 to be obtained per pistol. In order for the Gauss Gun to appear on the equipment list, it must first be researched under the Heavy category.