ORIGINAL: Star Ranger
I've been playing on normal and easy mode and still having money issues - I've been able to figure out avoiding bankruptcy - but other races have it way too easy to have larger empires and not have economic issues.
I'd recommend lowering the rebellion penalty for Humans and increasing the base economy for an empire - you shouldn't have to have trade missions to keep your economy afloat.
I have 120000 kredits and 0 rebellious planets with humans in my current second game on normal difficulty. Perhaps you are doing somthing wrong? I had bankruptcy too in my first game with humans but not after that.
ORIGINAL: Iceman
Last time I checked, trade missions would give you $3.5 per pop in the target system. They're only usefull in large pop systems of course, but then again, you don't want those worlds spied on - and so shouldn't the AI.
I have given most of my trade missions to my allies and so far they havent backstabbed me yet. I havent sent any mission only accepted and even then I havent accepted all. I didnt accept any from those I planned to invade in near future.
During first missions I even checked what planets they wanted it in. But since trade mission gives +1 happiness on planet due to trade goods currently I have started to accept almost all missions. Its a big bonus for humans.
I have noticed that the AIs somtimes ask trade missions second time to the same planet of theirs as before. It seems they deleted my trade mission on their planet or it was destroyed by other means and they want it back.