The TAP NAAA evaluator is supposed to give you an asking price in TAP.
Planes TYPICALLY sell for less than that now, as the market is soft.
Also, a valid comparision would be to compare the NUMBER of Archers in
TAP now, as compared to some time(s) ago, as this would give a TREND
(more now than yesterday means declining prices, because more are
trying to sell).

You will NEVER get a precisely accurate appraisal on ANYTHING because
each sale is different and the volume of aircraft is low, so low
liquidity. At best, you get an estimate. Most appraisals are higher
than actual sale.

Also, you might look in Ebay, under the transactions completed. Ebay
prices tend to be low, but if there are some Archers in there it does
give you some information.

Also, for partnerships, there MAY be value in the partnership itself,
above and beyond the plane. Is there a maintenance fund? If so, then
1/4 of that will become yours? So that would be included. Also, the
fact that there is an existing partners, existing maintenance procedure
and partnership paperwork is not trivial, and does contain value. And
of course you need to evaluate the quality of your partners (can they
pay their bills etc).