Alienware Area-51: http://www.dell.com/us/p/alienware-area51-r2/pd
Xidax X-8 in the XS case: http://www.xidax.com/desktops/x-8
Origin Millennium: http://www.originpc.com/configurator/prime/1.aspx?shellid=2
I've been pricing them out based on minimum acceptable specs in common: 5820K cpu, 16GB ram, GTX980 gpu, smallest SSD available. Based on that the prices only range by $300. The costs of better CPU or more GPUs vary significantly, but I plan to upgrade those later so it doesn't matter as much.
At this point, honestly, I think it mostly comes down to a) whether I care about motherboard specs that much or b) which case I like best. I like the X8's MSI X99 SLI Plus motherboard better than the Millennium's ASUS X99 Deluxe, but I can't really compare with the Area51, because it's a custom job with no real spec details.
Thoughts? I'm tempted to just go with the A51, but I fear buyer's remorse on large purchases.