I just took a quiz on CoD4 on Microsoft's Expertzone site and in it they confirmed that the map pack will cost $10, or 800 points. I kind of figured that I would be that much, but now we know for sure. There will also be a Game of the Year edition for the 360 that will come out soon and it will include a voucher to get the map pack for free on Xbox Live.
EDIT: Just noticed that it also says that the map pack will be released on Xbox Live on April 3rd. I have no idea about the PS3 but I would assume it would come out on the same day.
MS on GotY and map pack wrote:Call of Duty 4: Modern Warfare Game of the Year Edition gives gamers the chance to experience the intense cinematic action of the original game while expanding the online experience of 2007’s top-selling game by offering gamers a free token for the Variety Map Pack - four new multiplayer maps that will thrust gamers into a hot zone of combat across a variety of intense multiplayer locales. The Game of the Year Edition and Variety Map Pack will be available exclusively for the Xbox 360 for 21 days.
MS on price of map pack wrote:The Game of the Year Edition includes a code that allows consumers to download the Variety Map Pack, a $10 value, for free on Xbox LIVE Marketplace.
As you can see they dont directly mention the price. This is the only place I could find it.
MS on date of map pack release wrote:Call of Duty 4: Modern Warfare – Variety Map Pack
Ship Date: April 3, 2008 via Xbox LIVE MarketplaceCost: Those who purchase Call of Duty 4: Modern Warfare - Game of the Year Edition receive a redeemable token for the Map Pack. This redeemable token allows you to download the Map Pack from the Xbox LIVE Marketplace at no additional cost or Microsoft Points.