Al Kossow writes:
how rare are these? $150 seems like a lot of money to
spend on them
AFAIK they only shipped with a licensed copy of the 2nd Edition of
Plan 9, so they aren't that common. Sadly I lost my set a decade
ago :-(
$150 is probably a realistic value, given their rarity. Of course
Abebooks has completely destroyed the used book marketplace, so
it's impossible to accurately value anything that's out of print
these days.
--lyndon