The Torque Game Engine Advanced is one of the most economically friendly SDK kits for the creation of 3D simulations. It's wonderful if your like me and just starting out, as the development network has so many resources for just about any kind of game play you want. The Indie licence is only $250 one time payment, making it the cheapest for what you get. Also Marble Blast Ultra is a game which can be found for the PC & at the Xbox 360 Live Arcade and is a great example of what you can do with Torque.