Guild Wars' first (and probably only) expansion pack, then you might want to reconsider. Guild Wars: Eye of the North not only is available on Steam for the first time, but is being offered for 33% off one week only, bringing the price down to $13.39 until October 15th. This is a better deal than the discount that NCsoft is offering on its own store, where GW:EotN is on sale for $19.99.
While not a full-fledged campaign, Eye of the North does offer a huge assortment of goodies, including new regions, tons of dungeons, dozens of new skills, additional heroes, and rare armor sets.
For players eagerly awaiting Guild Wars 2, it's definitely worth a trip back through the lands of Tyria, especially now that we're learning more about how the Hall of Monuments from Eye of the North will "pay forward" rewards and titles into Guild Wars 2.