Unibody display assemblies are expensive; especially for split back unibodies.
Before I delve into this blog entry, let's first define what I mean when I say "split back" Unibody. A split back Unibody is a Unibody Macbook Pro from 2008, when the battery was user removable. Every Macbook where the back cover is user removable is a split back Unibody. These are not unibodies, they're dualbodies. You kinda got lied to.
The display assembly is comprised of two sheets of metal adhesed together. When the adhesive fails(it fails more often on the larger, heavier 15"), they separate, and do something similar to the image you see below to the glass.
15" Unibody Macbook Pro Display Assembly 2011
15" Unibody Macbook Pro Display Assembly 2010