
$2,500 per seat and no royalties to give. UDK cost $0 for non-commercial purpose, limited version, no access to source code, no possible to create your own shaders etc. If you make +$50,000 you give them %25 royalties. $99 for commercial project (individual), you can sell up to $50,000 without paying anything to UDK. UDK cost upfront is$0,limited version where you cannot have C++ access and cannot create your own shaders. Once was too expensive, but have changed their price to become the most affordable. $6,000 upfront for all platforms PC/MAC + iOS + Android + Flash + Unix + Web. $3,000 upfront for PC/'MAC/Web + one Mobile Platforms for commercial games, fully featured. $0 upfront for Unity free for commercial games! But limited, splash screen, no shadows, no full screen shaders, etc. supports many platforms with a price of $1500 per platform and no royalties if the source code is not needed. Up to you to decide what is best for you.
With Unity 4 and all new platforms each priced $1500 I wanted to just compare objectively the cost of each license for each competitors. All these engine can make games on multiple platforms.