|Element||Density ($g/cm^3$)||Value ($\$/kg$)|
|Plutonium||$19.84$||$4 000 000$|
It would be so easy if we could just substitute some of the gold for a cheaper metal, but it seems that gold is well-protected from this kind of forgery (which is perhaps part of the reason it's so special, to begin with). Almost all of the elements denser are vastly more expensive, or only producible in small quantities in a lab.
There is a very thin market for Rhenium, so we could buy some at a price lower than gold, but any more and we'll risk either bumping up the Remarket price or being foiled by the authorities. As thrifty criminals, we would like to go with Tungsten instead.
Can an allow increase the density of a metal? Qualitatively, I imagine that a lighter element could take up interstitial sites, and allow a higher density. But is there any known Tungsten compound that fits this, or any known method of alloying that would increase its density by that ever-so-slight amount?