Play are selling "Flight Simulator X: Acceleration" for the PC for £9.99 (inc Del.)
You can get cashback through the usual sites (e.g. 4% Quidco, making it just £9.59)
The next best price seems to be £17.95 from BlahDVD.
Flight Simulator X: Acceleration, is the action-packed expansion for Microsoft's amazingly successful and award-winning Flight Simulator X video game. With newly enhanced DX10 graphics, Flight Simulator X: Acceleration gamers of all ages, types and skill levels can race against friends in amazing, highly detailed locations around the world, test their skills and heroism in many new missions, fly three exciting new aircraft, and share rewards, photos and videos with friends and the broader flight community.
- Multiplayer Racing Mode: Flight Simulator X: Acceleration will offer the thrilling new multiplayer racing mode that will allow gamers to compete against friends with four types of racing including real-world and fictional Red Bull Air Race courses, the unlimited Reno National Championship course, as well as cross country, competition sailplane courses and fictional courses like the Hoop and Jet Canyon course. Gamers will be able to test their skills in three different levels of difficulty from simple pylon racing to racing very challenging courses in a variety of weather conditions.
- DirectX 10: Flight Simulator X: Acceleration will continue to build on the rich visual foundation of Flight Simulator X, taking advantage of Windows Vista and DirectX 10 to deliver breathtaking graphics and dynamic environments for a truly immersive experience. Use of DX10 graphics is optional.
- New Missions: Flight Simulator X: Acceleration will offer more than 20 new missions that will allow gamers test their skills at tasks ranging from a fighter jet test pilot to search and rescue.
- New Aircraft: Flight Simulator X: Acceleration will allow pilots to fly through highly detailed scenery in three new aircraft including the F/A-18A Hornet, P-51D Mustang, EH-101 helicopter.
- Connected World: Flight Simulator X: Acceleration continues to take advantage of Flight Simulator X's online game capabilities allowing gamers to race against friends as they interact with other aviators from around the globe by chatting in real-time via headset and keyboard.
- Games for Windows franchise: This expansion pack will also ship as a Games for Windows title with easier installation, reliability standards and support for key features of Windows Vista, including the Games Explorer and Parental Controls.