If your only goal from gathering is selling, follow demand. It's usually short lived. As Roth pointed out, gear might control that though.

I'm a slow grind gatherer, gathering for the sake of gathering and not leveling. A stack of Cloud Mica gets me 500K exper and while that's not brag worthy I need 11 stacks of it to justify all the Titanium ore and red star ruby I have in retainer. It has gotten me to 62 and half way to 63 mining so far.