Skydive is a game developed as a student project at University of Skövde in Sweden. We are 17 students that for ten weeks together have made this game in Unreal Development Kit. The gameplay mechanics are completely physics based, i.e. the character is really flying. The mountain is super huge (actually 14000 times larger than the Unreal Tournament map dm_deck). The game features several game modes, two characters and splitscreen support for up to four players. The game is a PC game but is played played with an Xbox 360 controller.
On UDK website, we can see this:
If you are using UDK internally within your business and the application created using UDK is not distributed to a third party (i.e., someone who is not your employee or subcontractor), you are required to pay Epic an annual license fee of US$2,500 per installed UDK developer seat per year.
So what is my point.
To make something like this we can use an free and better 3D engine.
Like Ogre 3D, Irrlicht.
Read this if you don't trust in my opinion.