Your email addy bounced my reply as recipient 
unknown -

Anyway - 
As a WK collector - I think the price will vary 
more according to which character you have, the 
scene/position and the quality or appeal of the 
rendering.  E.g., even the nicest depiction of 
Ken will never rate what an adequately appealing 
Aya will bring!
 
As far as fluctuations in value - the only major 
one I'm noticing now is with Ayashi no ceres. 
But's that's to be expected, as newer series take 
hold of the fans, and the resistance to buying 
sketches dwindles under the absolute lack of cels 
for CG shows.  Compared to some other shows, WK 
somehow keeps fairly level!