Well, a bit more probably. You need a case (cheap), some connector stuff (can be quite expensive), preferably a (pre-)-regulator, a diode to protect against input power connected the wrong way around, some random stuff.
I still think it can be done. Of course, BOM cost is not final product cost. But, assuming the usual BOM cost * 3, you could still maybe make something for ~$30. (I wish I had the time...)
I think I paid about 30 to 40 Euros for something using LT1021s with 5 and 10 Volts, 0.05% nominal. This is more than enough for the usual DMMs, and still enough to check that a 34401A is doing reasonably OK. The reason for having 5 and 10 Volts is that different DMMs have different ranges, checking near maximum range value is a good idea.
Yeah, mine has a 2V range and a 20V range. 2V is easy, of course. 20V... not so sure.
I don't think that is worth it for hobbyist use. Resistors with 0.01% cost real money. Precision OpAmps cost real money.
I dunno about precision resistors, but opamps with less than a mV of offset even in the worst case (typical = 200 uV) aren't that expensive.
LT1013 for $1.95 in qty 100, and that's two opamps per chip.
As far as I know for affordable DMMs you don't get to adjust every range individually, the ratios are fixed. So why bother?
I think hobbiests will buy all sorts of crazy stuff that's overkill, just for the fun of it. ;]
Though, again, I don't have the time to do the market research to back that assertion up. (Alas...)