1) You've picked an i7. The Mac Pro uses a Xeon. They're way more expensive. 2) The Mac Pro has ECC RAM which is more expensive. 3) You picked one video card, and it's probably a gamers card (not a GP-GPU like the FirePros, again, MUCH cheaper) 4) OCZ SSDs which have dodgy reliability and are SATA, not PCI-E 5) No Thunderbolt controllers 6) A 750-watt power supply - The mac pro gets by with 450w.
If you assemble a list of parts that are actually similar to what's in the Mac Pro, you'd probably end up closer to $5,400.
And none of that counting the desk space you save, the noise characteristics, the expandability (PCI-E speed and 6 hot-pluggable devices for each thunderbolt 2 port).
Where you're more likely to save money generally with Apple products is with upgrades, as some have pointed out with the Xeons.
