"Platinum Hits is a term used to refer to a line of select Xbox games that were considered by Microsoft to have sold considerable units on the platform in the nine months after release, and have dropped in price from their original MSRP to a newer, lower price, generally that of US$19.99, although multi-game packs may sell for more.
On September 8, 2006, Microsoft announced the Platinum Collection would be extended to the Xbox 360 platform. On September 20, 2006, at Microsoft's Pre-Tokyo Game Show conference, they announced Platinum Hits for the Xbox 360 in North America, priced at US$29.99 and Classics in the UK for GB£24.99. A second wave of titles was released in early 2007, with additional games being added periodically."
Source: Wikipedia, "Platinum_Hits", available under the CC-BY-SA License.