.stuff snipped
So, I have another question. This time, not how to compare airplanes, but
how to make an apples-to-apples comparison (money-wise) between different
specimens of the same model for sale.
.lots of stuff snipped

This is the $64,000 question. Many will give some specific examples, but
it should become apparent that this is a bit art and a bit of science.
There are many differences between the factory planes themselves of many
particular models. Paint jobs, interiors and the like are also wildly
variable. Engine condition depends on several factors and not just time
since major overhaul ("SMOH"). Avionics suites can have what appears to
be infinite combinations. They all affect price.

In order to arrive at a reasonable value, you need to look at many
examples, know the various details, SEE the birds, and then know the
asking and ACTUAL selling prices. This is WAY more difficult than it
sounds. Geography is a big factor. How people report what they are
selling is another huge variable. Finally, while the asking price is
posted, you rarely get to know the actual selling price. Not only do you
have to see a lot of birds, you also have to KNOW what you are looking
for and what you are looking at. This takes experience. The average age
of this entire fleet has surpassed 30 years. Age and a bird's history
take way different tolls on them. This all affects price.

The various valuation services give very general pricing based on
certain assumptions, some of which they state in their service. The
pricing between them can be significantly different. Many will argue
about which one is more "accurate" depending on whether they are a buyer
or seller and which service tells them a price they want to hear.

The big complication in all this is emotion. Patience is key at the
exact time when "love" trashes all reason. It may take a long time to
educate yourself. Even if you know what you are looking for and have the
experience, it may take a year to find that exact bird if you are
inflexible and precise in your requirements/cash or that make/model is
rare to begin with.

Some contend that these are the very reasons you should use a broker. In
theory, this is what a broker is paid to do (weed out the junkers from
the cream puffs). In reality, brokers themselves are a huge variable.
Since their primary incentive is moving airplanes for cash, you may not
get a "good" airplane in the end. If you take someone's recommendation
of which broker is "good", that in itself is a giant variable (who knows
if the recommending party is credible - who knows if the bird they
bought is really a junker - and on and on). Brokers themselves affect price.

In the end, with experience, time, money, and good luck, you may find
what appears to be EXACTLY what you are looking for. In time, you may
discover whether this is true or not. Since you cannot disassemble the
entire plane to inspect it, there are many opportunities for hidden
defects lie in wait. Defects you discover before the sale may move the
price a little or a lot. This is a big variable and it also may affect
price.

What's a plane worth?? Whatever someone will PAY. The most difficult
thing to do is to determine the difference between a set of reasonable
compromises and when to walk away.

Finally, if you take the time to do it right, some impatient fool with
more money than brains will likely buy the beast up from under you while
you are doing the research. Because the fleet is shrinking, there are
fewer and fewer choices. The market is reported to be "soft" right now.
So, prices MAY be more flexible on some birds. Everyone's experience is
so very different, it is hard to tell who to believe. There is no
service with a guarantee (you satisfaction or your money back is NOWHERE
to be found in aviation).

Good Luck,
Mike