The amazing Portal gun you see above is from toy manufacturer NECA (National Entertainment Collectibles Association), and isn't one of those one-off things. It will be available for purchase from various retailers sometime soon. The estimate is May.
The replica Portal gun is a 1X1 reproduction of the device from the game. It features blue and orange lights, and is limited to only 5,000 made.
I'm not sure what a "good" price is for this, but web offers I've seen have ranged from $124.99 to $206. I have a feeling these will be a lot more expensive on eBay a few months after the limited run sells out.