In a comment I writed what Reto did so I am not gonna write it 2 times. There are 2 ways of gaining money:
Go playing infantry:
- take an Assault Rifle or a SMG and go to the points. Also, dont fully-mod your weapon (it is not worth it, just a little more of RPM or damage is enough).
-Go to the points, (play the fakin objective / ptfo). Each capture and defense-kill gives you tons of Cs.
-Go by car. Each car costs only 25 Cs each (each moto costs 75 Cs), and put a health crate so you survive longer. Also I recommend you to use the first aid kits for surviving longer (each use of them only costs 24 Cs). For unlocking a car you need 42.000 Cs and driving the cicilian truck for some time.
- play skirmish or assault (in any map, dont put bias filters because you only like one map, if you put those stupid filters it will take you a lot longer to enter in matchmaking).
- dont use AP (Anti-Personal) grenades, each of them costs 150 Cs.
Dominate the skys
- each plane gives you a decent amount of money.
- buy your pilot and kill the enemy planes.
- you will win most air battles if you play with a good pilot friend, also.
- learn the spawns and you will became an ace (I don´t like that in-game easy way of becaming an ace but it is like it is ...)
- play in leveled battles and NOT in war (in leveled battles you are guaranted to have fun but in war only pros go and you waste resources if you arent experienced enough).
- if you arent a good pilot and you get wrecked take the other way :/ .... xD.

The maintenance cost
I have known this web from quite a while and I want to share it with you now I have updated this guide
https://docs.google.com/spreadsheets/d/1TO3th5nfLrhqx-HQDA8cGxD4zoarQT7nHozZUhgSTYA/edit#gid=0