With Fluke it is largely age: they introduced the 8840/8842 5.5 digit meters sometime in the late eighties, then nothing apart from the Fluke 45 for a long time, and they bought Datron and introduced the 8508 (8.5 digit) and 8846 (6.5) digit. So obviously the 8846 is much more expensive used than an 8840, 8810 or 8800.
If you look at the older Fluke 6.5 digit meters like 8502A, 8505A, 8506A, they can also be found for around the $100 mark depending on condition and options. Keep in mind that almost everything on those meters was optional except DCV, so you might end up with a meter that can measure nothing but DCV. There are also 6.5 digit meters of similar vintage from other brands like HP (3455a, 3456a, 3457a), Datron (106x/107x), Solartron, Prema and Keithley (196) that are available for well under $1000. I have not kept track of pricing, but a number of years ago it was common for the HP 3456A/3457A to go for about $100 in decent condition. Even the more recent HP/Agilent 34401A and Keithley 2000 can be found for around $250 if you are patient.
So if anything, I would say that there is not much price difference between 5.5 digit and 6.5 digit of similar vintage. More than 6.5 digit gets expensive because there are is less competition, but some 7.5 digit models like Datron 108x and Keithley 2001 can be found for under $1000.