They're full of platinum; well, maybe "full" is an exaggeration. Good quality platinum fetches £30/g, and low-grade is £11.50/g so you can see that it doesn't take much to make them quite valuable.it was new anth, that was worth a lot more than that, we get £40 as scrap for them so the guy you sold it to must have been rubbing his hands together
I had one apart recently for analysis, so can probably find the figure in mg/cm2 somewhere.