Microsoft has revised its policies for Xbox 360 achievements. From now on ALL games must include a 1,000 gamerscore in the core of the game, so that means you won't have to pay for additional content should you wish to achieve the full 1,000 points.
"1. All regular disc-based games MUST have 1,000 Gamerscore in the base game. This means that any consumer who buys a retail game will have the opportunity to unlock the full 1,000 Gamerscore without having to pay for any add-on content. Note that a publisher may decide to deliver a portion of this Gamerscore via add-on content, but the add-on content will always be free to the consumer."