Mac Pros are pretty low-performing machines for the price. They're outdone by the new iMacs in many benchmarks. Besides, to build an identical computer in PC would take about 1/5 the amount of money as buying it prebuilt from Apple.
I have to disagree with this. My Mac Pro runs circles around my friends brand new iMac. You have to remember that Mac Pro's have many more cores and the programs people who buy Mac Pro's use take advantage of these cores.
Programs like Maya, Real Flow, Houdini, After Effects, etc all run much faster on a Mac Pro. I've tested this when I went to visit said friend and had to install my software on his machine for 3 weeks. Although the iMac can perform faster for certain programs, it definitely cannot perform faster for all programs, including a majority that Mac Pro users are probably buying.
For people doing lighter stuff like Photoshop or Illustrator they will probably see an increase of speed on the iMac.