I agree totally with the previous post (hey he 
does it for a living), the Lithograph is printed 
in a series of plates (kind of like printing 
money) the plate gives a solid colour, where a 
poster is generally printed in a series of a 
combination of dots of verying colors (much like 
a bubblejet printer only has 3 colours but the 
combination makes many colours). The difference 
is in the final product, when viewed at a high 
resolution the poster style of printing resolves 
to the individual colours, the litho does not. As 
for the value of the litho, well that depends on 
who did the original artwork, and the production 
run, there is no reason why they should be priced 
any differently from any other limited edition 
artwork (its not the original, and neither is a 
repo from Princes Mononoke, yet they fetch over 
$300).