I've just arrived as a lvl 50 sith sorcerer and i know that you can get Rakata gear by running hard and nightmare mode operations but can you get it on Galactic trade network, and if so how much do it usally cost?
Unfortunately, no, you can't get Rakata Armor and weapons from the GTN. They're Binds on Pickup, and permanently bound to whoever picked them up when they drop from the HM or NiM Operations and I believe also Lost Island HM (please correct me if I'm wrong). Most of them also, if I remember correctly, are bought with Rakata item tokens, which are what actually drop from those Ops and Lost Island FP.
Honestly, unless you're going for looks, you're better off sticking to Columi and saving your Black Hole Commendations until you can get a full set of Black Hole gear. Then just move the mods over to the Columi set, and then BOOM! Instant Campaign set.