I think what happened there, is that Apple priced the Mac Pros when the Core 2 Quad Xeons came out, and hasn't reduced the prices since. Those chips are now a whole generation out of date, the chip price has gone way down, and Dell has followed it but Apple haven't.
The Xeons are pretty stable in their pricing. My Mac Pro has two Xeon E5462 chips in it. When I bought my Mac, I debated between dropping the price by $500 to only have a quad core. Back then, the price of one of the processors retail was ~$850. Checking today, 9 months later, they are still ~$850.
Apple does traditionally do what you say though, in that they don't reduce the price over the lifetime of the product. They just simply replace it at some point. The iMacs, Mac Mini, and Mac Pros are all due for an update sometime soon. The Mac Pro is held up at this point by Intel, assuming Apple keeps the Mac Pro as a workstation using Xeon processors.