Interesting discussion. My hypothesis is that the vast majority of guitars are not an attractive financial investment.
When I looked on Google Scholar for studies on this question, I did not find anything on guitars, but I did see a couple of papers looking at violins as a financial investment. One of these papers stated that the S&P 500 has had a real return of 6.3% from 1875 to 2012. A data set of 320 old Italian violins (Stradivari and the like) was analyzed, and the real return of the old violins was 3.3% for time period from 1875 to 2012. The paper can be found searching for "Fiddling with Value by Graddy and Margolis".
I would argue that it's hard for a guitar to appreciate in value at 6.3% per year to beat the S&P 500 benchmark. I'm sure there are some exceptions for instruments of historical significance in near mint condition... but not many.
For fun, we could look at a Kohno 30 from 1975. A Kohno 30 had a list price of 300,000 JPY. That list price translated into USD 1,016. (In 1975 about the exchange rate was about 295 JPY to 1 USD.) If I had taken that $1,106 and invested it in the S&P 500 in 1975, I would have about $13,234 in 2017. I see a Kohno 30 from 1975 on the large auction site that had a best offer accepted under $5,500. So while the seller of that Kohno 30 1975 probably enjoyed that instrument a lot, in purely financial terms other investments outperformed the Kohno 30.
Cervantes Concert Rodriguez, '08 (C/EIR)