Unreal Engine used to be licensed that way, but they've moved to a royalty model that actually makes it pretty cheap for small studios, and free for hobbyists. The current licensing options are,

Non-commercial use: free

Commercial use for in-house applications (e.g. training simulations): $2,500 per-seat

Commercial use for end-user-oriented applications (e.g. game development): $99 plus 25% of royalties above $50,000

Full terms: http://www.unrealengine.com/udk/licensing/commercial_license...