Yes, the improvement of chip technology led to economies of scale and reduced prices for quite a while. I haven't actually tried to swap the chips, but when Fluke introduced autoranging, they started putting the same part number C2506 DVM chip in their DMMs - if you can trust the Fluke part number, it's in both the 4.5 digit 8600A and the 5.5 digit 8800A. The schematic tends to confirm this. The way they operate the display, it makes sense that the same chip could be used for both - the actual accuracy was determined by the ADC anyway, so technically I guess they could have made the 8600A a 5.5 digit meter with 4.5 digit accuracy by adding another LED/driver.
The 8000A has a totally different set of internals - it uses a voltage-to-frequency converter instead of a dual-slope ADC and has a pair of custom ICs to implement that while the range switching stuff is mostly discrete. So it's kind of the complement of the 8600A where the ADC is mostly discrete. Must have been cheaper to do that, plus it's only 3.5 digits.
Since I was stuck at home in the snow today, I did an impromptu check of my bench DMMs against my 3 voltage references and my MC-7 resistor "standard". Considering that none of the Flukes have been adjusted in a long time, they held up pretty well; there was very little to choose between the 8800A and 8600A units on overall DCV accuracy. I wish I had actual measurements of my MC-7 though, instead of just the accuracy spec - it's really unknown which meters are closer to the accurate value and which are a victim of the resistors being slightly different from the marked value (the eternal metrology dilemma). Assuming the resistors are spot on, all of the DMMs beat 0.03% and all were very linear over the full range of resistance. In fact, my $10 8600A converted from battery to line operation "seemed" to be more accurate than the 8800A units, within the limits of resolution. I wouldn't hesitate to use any of them for bench testing.
40 year old meters. Great stuff.