In MWLL, you have your "base" pay (49k for the lowest rank), which means whenever you die and respawn, should you ever have lower than your base pay, you will spawn with it. Higher ranks have higher base pays (55k for warrior, etc). You earn rank via kills, damage, and objectives (if you are playing TC).
Ways you earn money in MWLL:
1. By far the best way is damage. In MWLL, you earn cbills per damage you do. If you do enough damage and survive long enough in a single asset you might find yourself with over 80k to play with, even if you might still be the first rank. If you die with said 80k cbills, then you should respawn with it (assuming your base pay isn't higher than what you died with). Though generally it's better to hike it back to base and sell (press J while in a vehicle) if you want to "upgrade".
2. Your starting/base pay. The more points you aquire (through damage, support, objectives, kills, etc) the higher your rank will be. Which in turn, means you spawn with more money should you happen to die when broke/poor.
3. Kills. Killing an enemy gives you a nice bonus, depending on the enemy. An Atlas kill in an Owens will be much more valuble than killing a Battlearmor for example.
4. Support. C3 can earn you cbills for simply using it with active radar, as will using NARC and having friendly missiles home in on your beacon. Using an APC and having BA spawn inside of it and your allies rearming themselves near it will also earn you cbills (you don't have to be in the APC the entire time AFAIK as long as the vehicle was purchased by you [and I think if you don't die]).
I didn't see warhead for 4.99? is that still going on?
It was a promotion on steam for one day, it's over now though.