* Quad core CPU
* >= 16 GB RAM
* >= 1 TB SSD
* Retina display with resolution at least as good as the MacBook 15"
* Battery life at least as good as the MacBook 15"
* Weight at least as good as the MacBook 15"
With those requirements---if you can meet them at all, which most can't---the price gap shrinks substantially.
The Razer Blade, for example, is $2700 for the QHD+/1TB option, and it's not obvious if it can meet the last two requirements.
The Alienware in a similar configuration is $2600 depending on the graphics card but the best screen option is 1080p and again, not obvious if battery is good and the weight is just a joke. No amount of money is going to get me to carry a > 7 pound laptop.
And that's without going into more subtle issues like build quality, track pad quality, etc.