Do you mean original Star Wars art work (like film story boards or cartoon cels?) or something like blueprints for toys/packaging art work?
Original art work like the above examples can range from a tenner (an ewok/droids cartoon cel) to big bucks (any original artwork for the Vintage toy line for example could be worth a lot of money)
You can see a lot of original artwork in this thread - http://forum.rebelscum.com/t1037817/
Perhaps have a flick through and if something looks familiar to your own item(s) you can contact the members there to find out about values?
Hope that helps a little, it's not my area of expertise at all but all I would say is any custom art will only be worth as much as the original commission (and probably even less when buying second hand) and reproduction/reprints are virtually worthless.
*If it's none of the above then feel free to open a new thread with pictures and some information and perhaps the guys can help out!