I am interested in knowing the demand curve for coffee purchased at a
coffee shop.  For example, the information provided should allow me to
determine the marginal value of another cup of coffee during a given
time period, such as a week or month.  The answer will have *nothing*
to do with the price of commodity coffee or coffee purchased in the
supermarket.

This may help you http://www.mbs.edu/home/jgans/mecon/value/Segment%203_3.htm

Bit of math that can help you work it out and also;

http://www.cabrillo.edu/~dambrosini/188Web/classsessions/pricing.htm 

If you scroll down a bit it shows you