I'm having trouble finding anything "official", I think I may have just seen them in comments here and elsewhere. It's all a bit imprecise as well, since not all of the components are completely clear (e.g. the GPUs are some custom Apple-specific model, which appears to have a generic PC equivalent, but it's not completely solid).
My vague recollection is that the GPUs alone are something like $700 each, and you get two, and the rest goes from there. Depending on the assumptions you make, you can get a $3,000 Mac Pro costing more than $3,000 for an equivalent PC, or less. Just beware of people comparing the Mac Pro with builds using non-Xeon CPUs, gaming GPUs, etc. Not that there's anything wrong with that sort of configuration, and personally I wish Apple offered some middle ground with high-end consumer components, but it doesn't give a good guide to what this actual machine costs to put together.
My vague recollection is that the GPUs alone are something like $700 each, and you get two, and the rest goes from there. Depending on the assumptions you make, you can get a $3,000 Mac Pro costing more than $3,000 for an equivalent PC, or less. Just beware of people comparing the Mac Pro with builds using non-Xeon CPUs, gaming GPUs, etc. Not that there's anything wrong with that sort of configuration, and personally I wish Apple offered some middle ground with high-end consumer components, but it doesn't give a good guide to what this actual machine costs to put together.