FIFA claim that the cup is
made of solid 18 carat (75%) gold
but Professor Poliakoff from The Periodic Table of Videos has been looking at data for the trophy and, using the density of gold, he reckons it is hollow.
httpv://www.youtube.com/watch?v=GuCgElVlZhk
The wikipedia page the professor mentioned is here.
Interestingly, the professor’s calculator looks almost as old as mine.