Hi Mark,
Thank you for your question.
You should be able to access the trap in the drain under the sink. You will see a "U" shaped portion of the drain and this can be removed. Below is a link that will give you a step by step for doing so.
Good luck and thanks again,
Josh
http://www.ehow.com/how_2327_retrieve-valuable-dropped.html
C&onstructi&on and Mechanical
Construction / Remodeling and Related Fields / Mechanical Repair