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.
True. I run UT to gear alts and not to make credits; there are so many better ways if you are looking for credits. The level 3 Xonolite as well as the Titanium and Quadrinium fund my level 6 missions when I feel like running them. I never sell UT purples. I run a lot of slicing missions and get enough hits there on UT level 6s that I can keep myself in Mando Iron and never buy them or have to run level 6 UTs. As I said, I do run the level 6s every so often when I am in the mood and alhough I do get Mando Irons there, I end up choking on Ciridium which I sell off cheaply just to relieve bag clutter.