I will have to disagree that "mitered corners" = "Quality built accordion".
I have both Lyra and Breaver brand vintage accordions with mitered corners. They are very cheaply made from what looks like pine box wood. Slapped together, almost "toy" quality.
They are not just "old" they were cheaply made when new and it shows. (But they do have mitered corners! :)
There is not that much difference in price if any as you mentioned. It's just a matter of going to the right builder. Now you do have to keep in mind that any of the particular builders who use mitered corners may also do many other things to make the whole a more professional instrument than just the mitered corners. Consider that also!!.
A picture sometimes speaks volume
Mitered veneer on those Chinese boxes. Not so inside.
When I set out to acquire a new instrument I did a little research. Went through the archives of this forum extensively, e-mailed and called a few builders in LA, but not all, and talked to all the accordion players who played at the CFMA dances in San Antonio. Everyone was proud of their particular instrument, but not one of the players or builders touted the type of corner joint as a selling point. Reed types, bellows, button springs, etc, were mentioned, however. Got one quote from Larry Miller, either verbally or e-mail, can't remember, for a C unit for
$ 1,500.00 or so. I have seen one of his instruments and it has mitred corners. Lots of builders did not respond, but this was during Katrina-Rita time. Got a quote from Danny Dyson for $ 1,000.00. He built it,I bought it, it has box corners, and it plays just fine. My brother builds quality furniture and I asked him about mitre/box corners from a strength perspective and he said it was all a cosmetic matter of preference.
And something else. This 'small industrie' is also a seller's market. With the increase in the popularity of the music type there will be more demand. One thing about any and all the builders in LA, they all build to quality, i.e., their reputation and intergrity are represented by their creation, irregardless of the type of corner joint they use. It is a credit to them that they do not 'cut corners' and mass produce a bunch of junk.
Don't forget about Jude's Bon Temps in Texas. I know you included him generically with LA boxes.
Just because Port Arthur, Groves, Orange, Beaumont, and even Winnie are on the West side of the Sabine River, that only means that they are geographically in Texas. Culturally, they are part of Louisiana. The fish in Sabine Lake swim to both sides and never know the difference.