Before they released the refresh (and prices are said to remain roughly the same), I had a machine spec'd out at exactly $2,600. 27" with everything upgraded except for the HDD. Same price for a 15" MBP (not retina) maxed out. I see their massive margins within the SSD upgrades, but then again I'm not an expert on computer tech by any means, so there are probably more variables that I am unaware of.
As for building a powerful PC, are there any all-in-one kind of options similar to the iMac? A major selling point for me on the iMac is that it is a single unit. I transport my workstation periodically, so I am planning to get a large pelican or similar case to pack the computer into for car transportation. Adding a tower + monitor dynamic to this equation would seem very unreasonable (yes, moving a desktop is less than ideal. Yes I have considered and dismissed the idea of a laptop).