To easily earn money at the mission where you have to destroy the large gun across the river from the Eiffel tower, go to where the Anti-Aircraft gun is located and take out all the guards. After completing the mission, use the AA gun to shoot down the Zeppelins. You will earn $50 for shooting down each Zeppelin. Shooting down the Zeppelins will raise your alarm level to 5 and cause airplanes to strafe you. You will earn $100 for each airplane that you shoot down. The Zeppelins and Aircraft will continuously respawn allowing you to earn as much money as desired. Once you are done, either save the game and reload it or jump off the building and die. You will however lose your weapons, but can use the money earned to replace them.