Trekkies will pay anything to grab artifacts from their favorite show, especially when the item is particularly unique or rare.
The one-of-a-kind phaser rifle only showed up in one episode of Star Trek, and that also happened to be the second pilot, though it's image also appeared on a Star Trek lunchbox (I didn't have one of those, unfortunately).
As extreme as $231,000 sounds, it was far from the highest auction price ever paid for a Star Trek collectable. In 2008 a collector paid $304,750 for the captain's chair from the Enterprise set, and a miniature model of the Enterprise went for $576,000 in 2006.
Related links: Rare 'Star Trek' Phaser Sold for $231,000 at Auction