Money Without Cheating
At Hogs & Cogs on the west side of Stanton you are able to do the "Bike Salesman" mission. This mission is exceptionally easy. After you sell all 40 bikes to complete the mission just continue to sell the faggio's or pcj-600's. For every five sales you get a bonus of $2,500 this amount accumulates. For example: at 200 sales (only took like an hour) you get $100,000 then at 205 sales (like a minuet later) you get an additional $102,500 and so on and so forth. I did this for like two hours and now I have a couple million and I've only completed like 25% of the game.

Extra Mission Rewards
Taxi Missions Level 12 - Unlocks Bickle '76
Ambulance Missions Level 12 - Infinite Run
Vigilante Missions Level 12 - Increases Maximum armour by 50%
Firefighter Missions Level 12 - Player is not-flammable (explosions still do damage)
Trash Truck Missions Level 12 - Money