https://docs.google.com/open?id=0B1_gNYG-sjMoUnc3Nk5qQXhRWGFIRXZ2b0pvUHVUZw
First post on here, got it to work. The formula isn't dynamic right now, in that you cannot change the quantities, but it could be done with a little more time.
Let me know if you have a problem viewing the file...