Best way to earn money?

[CombineSoldier]

Hello,

What is the fastest way to earn money in Alien Hallway?

I already got 5 stars in all hallways.

[CombineSoldier]

Nevermind guys, I found a way.

Always upgrade Gold in the Shop and always play the final level "Final Step!"

Also remember to earn gold stars in all other levels!

If you're having a hard time completing the final level, upgrade the engineer and the Heavy Laser Gunner.

[CombineSoldier]

Wait guys, you can play in the first level. Even through it says Level Gold: 450 you still earn way more. I get 5k each round.