Prey, as you may recall, was one of the early 360 exclusives (if you ignore the fact it was also available on PC, as Microsoft tend to) and, as such, was quite heavily hyped. The unique selling point was supposed to be the fact that gravity is not constant in the game meaning that rooms will "flip" as you walk across them with the floor becoming the ceiling and so on.
The reality was a little different. The plot and voice acting are terrible and borderline racist (you're a Native American captured by a vast alien spaceship out to escape and rescue your girlfriend by using the mystic powers which all Native Americans obviously have), the weapons bland and, because of the complex geometry of the rooms, tactics are non-existent (see enemy, aim, keep finger on fire until enemy is dead pretty much sums it up). Even worse is the mini-game when you die. Instead of sending you back to the last check-point or save point you have to spend several minutes hunting balls of energy with a bow to resurrect yourself. Tedious barely begins to cover it.
Overall, then, Prey along with Quake IV, Perfect Dark Zero and Kameo is one of the early super-hyped 360 titles which should be quietly forgotten. I assume that the reason it's so heavily discounted now is because it was massively overproduced at the time. If you really want to get yourself a 360 port of a PC FPS then it's not bad but you'd be better off checking out F.E.A.R. which is also heavily discounted and a much more enjoyable game.