Expensive but for a reason, that reason being exceptional quality. Naples yellow, also called antimony yellow is the chemical compound lead antinomiate. It is also known as jaune d'antimoine, one of the oldest synthetic pigments, dating from around 1620.
This is a darker counterpart of the two Lead Antinomiates, which is closer to the shade offered by makers in more recent times.
A classic shade, great for low flesh undertones, for fine use of it in this context have a look at Lucian Freud's paintings.
Its chemical composition is Pb(SbO3)2/Pb3(SbO4)2!
Very Fast Drying. Opaque. Excellent Lightfastness. Very Low Oil Content.