Apple announced newer faster Mac Pros today. They start at $2799 but you can configure them up to several thousand dollars (including software and accessories).
The really expensive bits are the 32 GB of RAM ($9100), the NVIDIA Quadro FX 5600 video card ($2850), the four 15,000 RPM hard drives ($800 each), the two 30" Cinema Displays ($1700 each), a Fibre Channel Card ($1000), and an unlimited-client copy of Mac OS X Server ($999).
That's a lot of money but you've got to remember that in addition to satisfying your computing needs well into the next decade, this baby will heat your entire house and provide a metal cooktop surface hot enough to prepare meals on. Mmm, 15,000 RPM omelettes! (thx, jake)
Update: Wow, configuring the new Xserve is even more expensive; adding all the possible options runs the price to over $83,000, which includes a $12,000 RAID array and $50,000 Mac OS X Server software support. $50K for support? Does Jobs come fix it himself?