In general, sketches vary in prices, depending on 
the details, color / b&W, paper that drawn 
on...etc many factors.  I have seen a couple 
detailed b&w sketches for Fushighi Yuugi went for 
$500 to $800 before.  

On the other hand, if you get a quick sketch from 
some no-name artists, they can worth as low as 
nothing, it really depends on the popularity of 
the artists.