In my mind, with the release of the Playstation 1 came the birth of the press kit. There isn’t a clear definition of what a press kit is, but most collectors agree that it should at least contain a (promo) copy of the game. These kits are sent to journalists and reviewers to present their new game in a special way and they often contain amazing extra’s that were never available for the average customer.
The rarest kits aren’t necessarily the most expensive ones, and the first Playstation 1 kits are extremely rare but never fetch big amounts. Lately, current gen press editions of big series such as Uncharted and Bioshock command a high price.
Nowadays a lot of brochures and promo items get sold on eBay as “press kits”, but only a select few games get a real press kit release.