You can buy cortosis on the GTN. And you can buy it off the crew skill trade vendor for $400 each.
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.