You're looking at $2,000 for the case, power supply, board parts/backplane, and heat sink for a single processor Mac Pro. If you add in all the other official Apple cables, screws, fans and what not that's at least another $500. If you are going to be buying parts retail then it isn't worth it.
There are better ways to get a cheaper Mac Pro. Look at refurbished or used models. 2009 models can have their firmware upgraded to use any LGA 1366 Xeons and you can flash a graphics card to save some money.
Look at
http://www.macpalace.com if you want to see for yourself.