Well, the Metroid Prime series is not like Halo. The DS game is actually a bit different, as well. The Prime series on the cube is a first person action adventure game, with some shooting in it. Rather than controlling the game with one joystick moving and straffing and the other looking around, you move around solely with the first joystick and the second is used to switch weapons. You can only look around by standing still and holding a button down.
Like I said, the game is more of an action adventure than a shooter. There is some platforming, and alot of puzzling. If you like action adventure and sci fi, I would definitely recomend the game. The graphics are beautiful and the story is interesting. Just be ready to recognize it as not a shooter. Prime and Prime 2: Echoes got rave reviews. http://www.gamerankings.com/htmlpages2/589573.asp
$9.99 sounds like a deal to me.