I'm no expert, but from what I gather, these would be the key factors.
1) age - if the necks still attached
2) size - some small bulby roots seems more expensive than big long roots.
3) shapes - thick bubbly roots better?
4) color - darker color better than pale color?
5) skin texture - the dense the rings the better
6) weight - I think heavy is not necessarily better for wild ginseng