I saw a copy of FS9 at Wall Mart for $19.00. Less than a night at the movies once you figure in popcorn and cokes.
Whether it runs smoothly or not depends on your hardware and situation in the sim. In a complex aircraft with heavy traffic on approach to a detailed airport I get a slideshow. On approach to a small regional airport in a Cessna then it's perfectly smooth for me. Took some tweaks and a lot of work with my comp. but it's definitely playable.