Direct2Drive has Prey for $2.49. If you use coupon code holidaytreats the price drops to $1.99. I don’t think the D2D version is active able on Steam. If you want a copy that can be activated on Steam you can head over to GamersGate
and pick up Prey for $2.49.
Info from the Prey wikipedia page:
Prey is a first-person shooter video game developed by Human Head Studios and produced by 3D Realms. The game was initially released in North America and Europe on 11 July 2006, and later was made available on Valve’s Steam system on 6 November 2006. Prey uses a heavily modified version of id Tech 4 to use portals and variable gravity to create the environments the player explores. The game’s story is focused on Cherokee Domasi “Tommy” Tawodi as he, his girlfriend, and grandfather are abducted aboard an alien spaceship known as The Sphere as it consumes material, both inanimate and living, from the Earth in order to sustain itself. Tommy’s Cherokee past allows him to let his spirit roam freely at times, and gives Tommy an edge in his attempt to stop the Sphere.
Prey currently has an 83/100 Metascore on Metacritic.