Interesting question ... brings back good and bad memories :)
Probably $5,800 for an HP 3000 Corporate Business System (a 12-processor HP
3000/997).
it was a million dollar machine when new.
I bought it for $300, paid about $500 for delivery, and $5000 to get the
three-phase power installed for it.
I reinstalled MPE/iX (the disks had been scrubbed), and discovered that MPE
wouldn't boot with more than 10 CPUs active ... due to licensing
constraints. All of the HP documentation I could find said that the 3000
version of the hardware was limited to 10 CPUs (the 9000 version, running
HP-UX, allowed up to 12).
Turns out HP had apparently done an under-the-table sale of a 12-CPU system
to someone, and patched (probably one byte of code change) the original
copy of the OS to disregard the 10 CPU limit. (Under the table sales, and
support, wasn't unusual with HP near the end of the HP 3000's lifetime.)
The machine was in a spare room at our office in Cupertino. When we had to
move, I donated it to Paul Allen's Living Computers Museum, in Seattle.
Stan