Part of the reason that configuration is so expensive is the 1 TB SSD. It's only an option on the $1800 model and costs an extra $500 on top of that. On the baseline 15" it would cost $800 more to get the 1 TB, $500 more on the $2500 model.
Also, the display is a QHD (2560x1600).
I don't doubt it's a great machine, but admittedly the CPU upgrade is kind of pointless. If someone actually has need for the more powerful CPU they can get a 15" rMBP with quad-core and a 1 TB SSD for only $100 more. If they go for a refurb it can cost even less.
Or they don't want the larger machine. This is the fastest 13" Mac available.