I have no doubt that you can't build a equivilant mac pro for cheaper than you can buy a proper mac pro.
Actually, only the single processor variant was cheaper than the cost of the retail parts and that's without operating system or software.
However to be fair the open computer is not supposed to be an alternative to the mac pro, it's supposed to be an alternative to the mini.
Not too many who are in the minis market would be interested in a hackintosh. In fact, with the exception of the pitifully small laptop hard drives they come with, of them would be perfectly happy with a Mini (or sub-$1000 iMac with similar specs). Who this is really for is enthusiasts who want to switch to Mac OS X and traditional buyers of the $1299-$1699 PowerMac who had the run pulled out from under them by Apple. I settled and downgraded to a computer that doesn't meet my needs, but for some desperate times mean desperate measures.