Please see my November 28 entry for a discussion of memory costs for the Mac Pro and MacBook Pro and iMac.
The 8GB modules for the Mac Pro come at a premium; higher density chips are more expensive. Yet similar price trends apply to the 8GB modules as for 4GB modules, and that trend is down.
Mac Pro memory slots
The 4-core and 6-core Mac Pros have 4 memory slots and can thus take 32GB (but Apple won’t sell you 8GB modules). Top performance is with three (3) modules, because of triple-channel memory. See the tests.
Apple pricing vs non-Apple
Apple won’t sell you 8GB modules for the Mac Pro.
You can get 64GB at OWC for $2029.99.
Or you can get 32BGB at Apple for $4000 ($3250 as an upgrade when buying a new Mac Pro). That means Apple is charging 4X as much.
Given the total system cost of a Mac Pro, it is now the smart move to get the optimal 24GB configuration for a 4/6-core Mac Pro for about $770, and that price should continue to drop.