The PDP8/S is the rarest and, in a quirky way, the
most interesting, member of a series of machines that
was particularly significant and popular among
collectors. It is not surprising that it would be
highly-valued. It is also an earlier machine than
is often found today in restorable condition, using
discrete-transistor circuitry. The particular example
for sale is also very display-worthy, appearing to be
truly museum-quality. I wouldn't read too much to
much into this for the state of computer collecting
in general, though the older (> 25 years) and rarer
machines are only going to continue to increase in
value.
--Bill