After completing the first UP mission and receiving Ewan, talk to him about the Winch challenge. After completing level 3 on this challenge, bet the maximum amount of money. Even though you have already done this, the height does not raise. Therefore you can continue betting at the maximum amount. After about four times (depending on how much you start with), you should have about $7.0 million. You can continue from here in order to get the maximum wager amount to $5.0 million (when you have about $14.0 million in funds). From there you can repeat this as often as desired when you need money.

Note: To save time, if you are trying to use the above method to increase your money, use stackable items from outside of your stronghold. For example, try the gas canisters across the road (or for something less volatile, use the shipping containers directly across the lake behind your house). Items used from outside of your stronghold stay in the exact location you put them until you quit your game, giving you a prepared stack height the next time around. Two or three attempts may be required to gather the items, but it is well worth it, as once done it requires only seconds to win the challenge.