The problem with dougas themselves is that they
are, essentially, the line drawings for cels. 
Which means that they can possibly be drawn in 
multilayered settings just like cels (with 
different people drawn on different sheets, or 
mouthless people and several mouthes, or 
different body parts that only a few move, and 
so on), but, unlike cels, you can't stack them 
to give you the full picture.

That's one reason, I think, that gengas are 
usually worth more.  That, and the gengas look 
more hand drawn than dougas, and the correction 
sheets are hardly ever a full drawing either 
(usually just fixing a few lines/shadows here 
and there).

All my experience comes from dealing with 
Azumanga Daioh douga/genga though, so I can't 
say for certain that's how it is with all 
anime.  I would be surprized if it were terribly 
different though.

Also, I don't have any original storyboards, but 
I have photocopies of some of them.  They may be 
worth more to a 'collector', but visually 
speaking they aren't nearly as pretty as the 
dougas and gengas, which, for me, basically 
means its worthless.  If I don't enjoy actually 
looking at it at a non-historical manner, I 
really don't want it.