You put forth a gauntlet but people on the forums prefer to design high performance, not bottom price.
That's sad actually - that today engineers often are forgetting what innovative and excellent design means. They just search best components around, throw it all together using traditional approach/circuit - to get target performance ignoring that by using their heads, most likely similar performance can be achieved using less money, thou most likely using more design time. Good example where engineers were using their heads: EDC 4601 AC Voltage Standard (EEVblog #709 episode), check service manual page 24. There's 1ppm (!) reference using two zeners in elegant compensation circuit. Something like this is what I am looking here for
I started this discussion in hope that some can pick-up challenge of achieving as much as possible using as low cost as possible (as 7$ LCR/component meter do). Those who say "just buy used multimeter" missed essence completely. It's not my build, I just came to brainstorm. After all I own decent enough bench multimeter already.
Reference most likely is one component which is as it is. Possible choices are LM399 (obviously), then REF5050 (3ppm) and mentioned MAX6164 (5ppm). PCB can be made compatible to accommodate different references - layout for default ref and pins to connect daughterboard. Those who don't mind some bigger thermal drift, can put on that board reference they have in their bin.
Next comes ADC. I would like to discuss multi- or dual-slope converter here. I feel/hope that this is doable for lower cost than some decent 20/24 -bit ADCs.
Regarding resistors - what you say about putting some 50 or even 100ppm thin film resistors on daughter board so they are thermally coupled as much as possible? On this daughter board I offer to put NTC as well (to measure temperature of said board). - Those who seek for as much performance as possible - they can do two point calibration, at two different temperatures. This could possibly allow to cancel some of resistor TC drift-induced error. What you say?
Those who ask me to state not only resolution (5 1/2 digits), but accuracy as well - honestly I don't know. As good as possible for BOM cost which does not let you break a bank. I would like to see what we can come-up with. I don't want to state unrealistic requirements and because of that ruin all the discussion. 0.005% at TCAL +/-1oC? Doable? If this needs 50$ worth of resistor arrays, then what can be achievable when we spend just 10$ for resistors that gives let's say 4 ranges (50V, 5V, 500mV and 50mV)? Daughter board PCB can be made in dimensions compatible to some well-known brand decade divider - so again those who seek for best performance, can open their wallets and get it.
Obviously 20$ total BOM is just start of discussion. I would put BOM pain threshold at let's say 50$, but then most likely it will be not so desirable for masses anymore. Who knows..