Yeah I figured that, but there are those who would like the easy access to parts and upgrades the mac pro offers. If the screen fails on the imac(rare I know) you need to fork out.
Dual socket motherboards and Xeon chips are quite expensive. The Dual socket Mac Pro's are comparable in price to other workstations from Dell etc.
The Single-socket Mac Pro's are quite a rip-off though compared to a custom Quad core i7 PC. Of course you get excellent build quality, cool little drawers for your hard drives, lots of PCI-E slots, 1 kW PSU, very quiet operation and Mac OS X so it's worth it for the Pro market - people who spend R20-R40k on Software anyway.