this is just from my experiences
purple cost more because most "real" purple plants, "GDP, PK" usually dont yield A LOT. and a REAL purple bud will somewhat have almost a grape/bud aroma and taste, almost like a grape swisher with tasteless bud. its a pretty sweet smell, and not many other plants put off such a "sweet" smell, with exceptions of jack herer and NL strains.
on the street, people always charge more for purple, i honestly think purple is way overrated when your talking about the actually HIGH, you get some good green bud that is indica dominant, like some white rhino, and you will be on your ass longer than any purple.
the PK and GDP's ive seen grown were always green, but had that grape smell, but the whole entire thing was green, not even a hue of purple. not even on the leaves during harvest. yet the PK's tasted and smelled absolutely wonderful.
i heard that purple is lower temps during darkness, and nitrogen deficiency or phosphurous deficiency, which doesnt make too much sense to me,- lower phosphurous, less fuller buds. you would get something like... this
