If you want to earn some money without a hard work just ask people who live in your villa if they want make a deal with you. For example, if you speak with Fiona you will be have a chance to take part in competition; it is about destroying targets with machine guns and grenade launchers as quick as possible [#171].
Each challenge is limited by time. If you manage to destroy all objects in time whole accepted amount of money will be yours. If you lose Fiona will be rewarded. Use 'Change wager' [#172] to change the amount.
Each reward unlocks another level of wager. Higher level makes that you have less time to do all goals. But in exchange for that you can win much more money.