As with all musical instruments. Those at the mid to lower end of cost usually don't increase in value much if any, ever. If you are buying as an investment forget it. It just ain't going to happen. It's going to take a while before they become classics and they still may not increase in value. IMHO!!
There is not enough data on the number of accordions being built by Cajuns to make predictions.
But I will make this prediction:
As all other accordion brands gradually rise in price, the value of used Bon Cajun accordions should increase as well. Ten years from now, when the other brands cost $2600 (or whatever, higher prices than now) new, I bet that the Bon Cajuns will command much more than they ever did when new. So, I don't think that they'll ever be worth more due to some "cult value" (as 'Acadians' will have). However, they should increase in value, as long as they are playable, as new accordions will continue to cost more and more each year.
A lot will depend on the quality of future production and the availability.
I wonder how many accordions Larry Miller made, and whether he kept records of each one. If he made dozens (or even 100) or more per year, and demand remains constant, there sure will be a gap to fill!
I've been a follower of prices on used and new boxes for some years now.. I predict that new boxes will top the $2,000 mark within the next 5 years... used boxes will ( like Andy mentioned ) follow closely behind .. maybe $100 or $200 less. The make will be critical. Martins, Falcons, Millers, Acadians ( and many others we all know of ) will hold value and others only a hundred or two hundred bucks behind the price of a new box. Reed quality and wear will count heavily on value.. .. eBay is not a good judge of value..as it is a global pawn shop of sorts. Bargains can be found, but rarely is an instrument sold at its actual worth.
Older, minty, well played boxes are going to be sought after.. not necessarily brand new ones or beat up vintage brand name ones, and they will command nearly the same price as new ones.
We will all get pickier.. and wood,detailing, buttons will figure more in establishing value of used accordions than just brand name.