P.metallica is definately not the most rare tarantula, but it has a bit of a legendary tag associated with it. Species like Monocentropus lambertoni is much rarer, but will probably never command the price that metallica does.
They are stunning in person and don't think many people would turn down a metallica if they could afford it. As for the price, it will be a long time before the price here drops.
On another note, people seem to have a thing for blue spiders. Just look how versi's, lividum's and balfouri's sell.
Like I said, there will always be a few "popular" species that people would probably not turn down, and that I suppose is just how the hobby works. Like owning a ferrari.

I have 2 little blue pokies and they are stunning and wouldn't swop/sell them easily.

Definately a species worth having.