Ways to earn cash: Medals (1 cash for bronze, 3 for silver, 5 for gold), 250 kills in one room = 1 cash. I got a psg1 using this, and still have 14 cash left...of course I did do a lot of farming with my friends and lots of combos...shouldn't be that hard.
I think this is a glitch: If you have a campaign and upgrade your units in the menu, you can keep those upgrades when you make a new game. Doin the first mission w/ bombers now.
Btw, Starfighter: Infinity is coming out soon. It has Multiplayer! And way better graphics...it's just SF:DG x 10^10 with multiplayer... k maybe not that much better, but you know what I mean.
 "Viricide"){kind=icon}
 "You Find Yourself In A Room"){kind=icon}
 "Solarmax"){kind=icon}
 "Kingdom Rush"){kind=icon}
 "The Ocean Around Me - Week Two"){kind=icon}
 "TDP4 Team Battle"){kind=icon}
 "Chuck the Sheep"){kind=icon}
 "My \"Dear\" Boss"){kind=icon}
 "Squadz 2"){kind=icon}
 "Starfighter"){kind=icon}
 "Outpost:Haven"){kind=icon}
 "Red Remover"){kind=icon}