This week's Q&A: no plans to make purple and white craftable at this time, particularly white.
So definitely rare--an "I was there" reward to people who bought them pre 1.2. From the statement, we can surmise that BW is not intending to make them common, so if they do come back as a reward, expect it to be on par with the 2.5 million credit price tag they once carried.
Pretty much this.
White at least is one color they want to keep extremely rare. If you want one, I recommend banking credits somewhere for the if/when they put the vendor back in the game to buy one. Purple crystals, at least, can be found pretty readily on the Imperial side of things, it's the equivalent of the Republic's cyan.