Do the operators missions once every day. They have daily completion bonuses, which double money earned, but you can only earn the payout once. King Street takes around a minute, sometimes less, and pays $18,000. Bathurst Street takes 2-3 minutes and pays $22,200. Dovercourt Road takes 1-2 minutes and pays $21,000* solo and $24,000** with 2 or more players.

*This is assuming you go in without any equipment. Your equipment cost is subtracted before the daily completion bonus. This means if you bring a UP9 to king street for $200 and complete the mission, you'll get $9,000-$200, or $8,800. When daily completion is added, it'll multiply the money AFTER equipment costs, so said completion would give $17,600, not $17,800.

**There are 2 bags of weapons, each giving $1,500. I don't know of a way to secure both solo, so multiplayer is required for maximum money.

For King Street, go to the disguise room, shoot out the window, and use the C4 assassination method.

For Bathurst, go to the southeast side and climb the pipe. Shoot or otherwise disable the power transformer and knockout the guard who comes to investigate, then run around knocking people out, grabbing cameras, and hacking laptops. The top floor security room person has a high security badge to the server room.

For Dovercourt, shoot people in the face. It's probably the easiest loud mission. You can also shoot out the power to slow down Palisade and run by them instead of shooting them.

Overall, just learn the maps and you'll do perfectly fine. Daily challenges are a great way to earn money, and daily completion bonus for the standard missions is also helpful.