Yes, but you are paying a little less than double if you buy off the vendor. The moderate mission costs 2150 and you are all but guaranteed 10 (Yes yes there is a chance of failure, but you are just as likely to crit and get double so it balances out). that's 215 each. If you have the time you are FAR better off sending a companion on the mission.
As for the GTN, all you are doing is paying someone else to run the mission for you, so you obviously are going to pay more.
In all MMOs, time and in-game money are interchangeable commodities: You can save credits by taking more time and you can save time by spending more credits.
True. However the OP wanted to make augment kits faster. This is a way for him to do so. If he is selling the kits on the GTN, he should be smart enough to price them so he makes a profit even if he bought them for $400. I don't know what he plans to do with the kits, I'm just helping him get cortosis substrates faster.
And I agree that since cortosis substrates are grade 6 they are fine with only being available in moderate yield missions. If a change is needed, then either raise the price the vendor sells them for, or make them grade 5 with moderate to bountiful yield missions.