I cleaned out the Elvin city and the dragon is dead as well but no gold cup, it is not on his corpse. Did it twice is it stashed in the bushes or something I can not find it. looked every where...
Maybe you've got a bug. Try downloading a patch or somthing. And if you still don't find it. Then get yourself Shadowkeeper and give your PC the cup. And then you can still kill the dragon.
A life without freedom, is no life at all
-William Wallace