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.
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.
Also regarding the bet amount, once you go over $10 million, it only gives you $1 million and slowly increases.
Find a Cash Bundle out in the field. Make sure you are in an area with lots of open room. If not, call in a helicopter and take the money to a safe place (for example, the PMC air strip). Tag it and call it in. When Ewen picks it up and you are credited for the find, shoot the money crate. You can do this by any means necessary (for example, use a nearby armed combat chopper, wait for Ewen to get there, then shoot the crate after getting the credit). Money should rain down on the ground around you. Run around and collect it all for an added bonus of roughly $50,000. The bits that scatter across the ground are worth about $5,000 each.
Once you have obtained your helicopter pilot, he will supply a bet, wagering that you cannot stack enough things to get the toolbox above the displayed height. This wager is easy and the money goes up by about $2,000 each time you play.
