This is a game the pro-Apple press loves to play - cherry pick parts for the PC based upon comparison to the often unique part numbers Apple uses (and which they buy in bulk) and ignore arbitrage.

How much would a MacPro comparable to this Dell cost?

    Dell Precision T7600 
    $12,697.60 
 
    Two Intel® Xeon® Processors E5-2687W (Eight Core, 3.1GHz, 20M, 8.0 GT/s, Turbo+)  
    128GB, DDR3 RDIMM Memory,1600MHz, ECC (8 x 16GB DIMMs)  
    Dual 4.0GB NVIDIA® Quadro® K5000, Dual MON, 2 DP & 1 DVI  
    PERC H310 for Dell Precision, SATA/SAS 6Gb/s, RAID 0/1/5/10 (8 ports)  
    Dual 512GB, 2.5" SATA 6Gb/s Solid State Drive  
    6X Blu-ray Disc (BD-RE) Burner  
    Speakers Dell AX210 Speakers  
    3 Year ProSupport Service with 3 Year NBD Onsite Service after Remote Diagnosis  
    3 Year Accidental Damage Service

If the rules of the game were reversed, you'd have to buy two 64GB MacPros, an external Blu-Ray burner, and go to a third party for a three year accidental damage contract and a third party for three years of Next Business Day onsite service.

You should do the homework yourself, rather than relying on an article whose motivations you can't be sure of.

It's awkward to match the Apple machine exactly because of the non-standard graphics hardware. The D300/500 are approximately cut-down versions of W7000/8000 with reduced VRAM.

I specced up a machine on newegg and other sources for the CPU. E5-1660 V2 (3.7Ghz with 15M cache), LGA 2011 motherboard, 64G ECC RAM, 2xW7000 graphics, 480G PCIe SSD (2x240G), and a CPU cooler / case / power supply to round it out (not looking to buy the most expensive options here). Came to about $4800.

An Apple.com Mac Pro machine with 3.5Ghz / 12M cache CPU (worse), 64G memory, dual D500 GPU (worse) and 256G PCIe (worse) comes to $5200.

On the pro Apple side, you get a very nice case and a lot of integrated wireless stuff, Thunderbolt etc. On the anti Apple side, you get much better expandability (the option to go dual socket in particular) and upgradability, and an overall more powerful system for nearly 10% less.

For the whole machine, but a quick check of the configuration on the processor (and only the processor):

Base system options:

  3.7GHz quad-core with 10MB of L3 cache 
  3.5GHz 6-core with 12MB of L3 cache [Add $500.00] 
  3.0GHz 8-core with 25MB of L3 cache [Add $2,000.00] 
  2.7GHz 12-core with 30MB of L3 cache [Add $3,500.00] 

[1] seems to be equivalent to the 12-core option at Newegg. So purchased there it's $2750, a savings of some $750 (less tax and shipping) versus the upgrade option through Apple. The overall price is better than a DIY equivalent, but for the components this path can still make a difference.

[1] http://www.newegg.com/Product/Product.aspx?Item=N82E16819116...

You're mistaken. The savings is in the GPU, not the processor: http://techgage.com/article/apples-newest-mac-pro-costs-less...