Hmm, didn't try to do lower grad missions so far,
I've mainly done grade 6 missions (grade 5 rich ones for the companion gifts)
and these are either for the companion gifts: 2200 credits a mission(taking half an hour each)
and 1 hour each for the 2700 credit underworld metal missions.
Definitely try lower-grade missions. Specifically, Grade 3 underworld metals that provide Xonolite. Those usually go for 1,000+. Everyone is running Grade 6 Underworld Metal missions to try to get Mandalorian Iron, which is why the Grade 6 metals sell for much cheaper.