Anyway, you may be able to get a REALLY GOOD machine (even 'better' for some things,) for much less than a Mac Pro, but you won't be able to get an 'EQUIVALENT' machine for much less. The Mac Pro really is a good deal. (Assuming you don't buy extra RAM or hard drives from Apple, that is.)
The cheapest you can configure a Mac Pro is $2300 for a quad-core 2.8 GHz, 2 GB RAM, 320 GB hard drive, Radeon 2600XT, and a SuperDrive.
Yes, you can configure a roughly equivalent Core 2 Quad system for much less than $2300.
But if you want 8-cores, the Mac Pro is $2799, while the cheapest I can configure an equivalent machine is $300 for the motherboard, $850 each processor, $83 per 1 GB FB-DIMM, $65 for a 320 GB hard drive, $30 for a SuperDrive, $33 for a 2600XT. Then we need to add in a case and power supply. $200 for an ultra-cheap Extended ATX case with insufficient power supply, $200 more for a barely-acceptable server-grade power supply. We're up to $2694, and don't have a keyboard or mouse. Toss in a dirt-cheap $5 mouse and $5 keyboard, and we're saving $95.
Oh, and you don't have an OS. If you want to remain quasi-legal, Amazon has Leopard for $109.49. Oops, we're now at more than the equivalent Mac Pro.
Yes, it is possible to build Apple's missing 'headless midrange desktop' for cheaper than the Mac Pro, but it is not possible to realistically build a Mac Pro replacement for less than the Mac Pro. Don't try to claim you're building a Mac Pro equivalent, when you're not.