From TakaraTomy's website, are official images of a black/silver Unicron from Arms Micron, called Nightmare Unicron. It is to be released March 30th, at stores that had the Satellite display/program (A matrix shaped TV promoting Transformers).
It is 6,300yen (about AU$75), but is limited, so if you want one "cheap", you'll need to know someone in Japan who can personally buy one from a participating store.

The metallic colouring is amazingly well done... it actually looks like diecast.