The acronym stands for Powell Knife Pistol. It’s a single shot firearm + knife combination, unlike the G.R.A.D which is multi-shot. Comes in different calibers, ranging from .380 ACP, 9x19mm and up to .45 ACP. Last I heard back in 2011 (when I was going to buy one) the BATFE does not consider these to be AOW (Any Other Weapons), and they purchased and transferred much like a regular pistol.
Considered quite rare nowadays since they are no longer in production due to the high cost of manufacturing them. The price can vary depending on caliber. Seen some for $950, all the way up to $2,000; all before any of the panic buying late in 2012.