SoftUK is selling "Flight Simulator X Deluxe Edition" for £21.99 (inc Del.)
Please note that it is currently listed as "out of stock and will require a delayed dispatch", but as this is the cheapest I have ever seen the Deluxe Edition for sale, and £8 cheaper than the next best price (from Game), I thought the wait would be worth it. ;-)
Deluxe Edition Information from Wikipedia:
Flight Simulator X was released in two editions, Standard and Deluxe. Deluxe edition incorporates some additional features, including an on-disc Software Development Kit (SDK), three airplanes with the Garmin G1000 Flight Deck, and player Air Traffic Control.
In addition, the Deluxe Edition features 24 aircraft compared to 18 in the Standard Edition, 45 high-detail airports compared to 40, 38 high-detail cities compared to 28, and 51 structured missions compared to 30+.