You can get a decent amount of money simply by playing through the various missions and completing their objectives. If however you need extra cash there is an easy cheese that can be found in the fourth mission which takes place in Arakcheyev Fortress. This mission is set on an island in Siberia and will involve going against a doctor and her assistant in order to stop their eugenics program. Check out our Easy Money Cheese guide to find out how you can perform the run in less than 5 minutes and get 150,000 cash each time.
Before you attempt to tackle this mission it is advised that you first unlock the 'Sniper Rifle Turret' (gadget) and the mask's 'Tracker' perk. You may also want to consider if you have plenty of cash spare the 'Booby Trap' and 'Semi-Auto' perks as both will make things easier. Start the Arakcheyev mission by fast travelling to the northernmost point of the map and search for the cavern that is there. You will have no problem spotting the mines that are in the area if you unlocked the masks 'Tracker' perk. Take out the sniper at his perch who is guarding the cavern entrance.
One of the objectives of this mission is to assassinate Dr. Antanasia Alikhanova who you will spot along with a body double at the helipad. The first one you shoot will always be the wrong one and once alerted of your presence the real one will run inside the fortress and basically escape. You can avoid this happening if you have the Sniper Rifle Turret by tagging both Alikhanovas ('H') and shoot ('G'). With the 'Booby Trap' perk, the extra range should be enough. And, with the 'Semi-Auto' perk, the turret can quickly eliminate both targets. Alternatively, you can also do a tandem shot with your turret.
If you don't have the 'Sniper Rifle Turret' unlocked you will have no option but to be quick with your long ranged weapon. The distance of both targets will usually be in the region of 120 meters away from your vantage point. Once you have killed both targets you need to quickly return to the extraction point which will be the area you spawned earlier. On your way back don't forget to ping the map to spot the mines so you can easily get past them as you sprint back.
Check out our Character Skills and Perks Guide to see a complete list of what options you have available in Sniper: Ghost Warrior Contracts.
You can actually get a double kill with a regular sniper rifle if you work yourself to the left, off the sniping position and down one level, then aim for the midsection or hip region when the dopplegangers are closely aligned.