If you want one ASAP, go to a clone maker like Aspen Systems.
A functional 500MHz box (minus screen) starts at circa $3500 with
Linux or WinNT. You probably would want to soup it up a bit and get a
screen so count on $5K or $6K. This is a "PC164a" motherboard with
1MB cache soldered on.
A Personal Workstation (like mine) with new faster memory chip-set,
SDRAM and 2MB L3 cache can be had from DEC for twice as many pesos and
three weeks or so waiting.
If you can wait a month or two, there will be sub-$3K machines with a
new chip, the 21164PC, around. It's the same chip with bigger L1
cache, L2 taken off-chip (i.e., L3 becomes L2), motion-video
instructions and cheaper to make. Presumably a little slower for most
stuff.
If you have lots ($25K) of money AND you can wait, you can order a
600MHz 21164a version but they won't deliver it for a while. On the
horizon for Autumn are machines with a xxxMhz 21264 which will be
twice(ish) as fast.
-- Wub.