After completing the first UP mission and receiving Ewan, speak with 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 will not raise. Continue betting the maximum amount and after about four times , 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. Repeat as desired.
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. Items used from outside of your stronghold stay in the exact location you put them until you quit your game, offering you a prepared stack height the next time around. Also, regarding the bet amount, once you go over $10 million, you only receive $1 million at first and slowly increases.
Once you have acquired 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.
Find a Cash Bundle out in the field while making sure you are in an area with plenty of open room. If not, call in a helicopter and take the money to a safe area. 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 any way you want (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.