What about using standard logic devices and shange supply voltage to get the desired reference level.
Well, that will change the output as well and not be useable. Using the supply voltage required for the output, if you can get the input in range the HC(T)14 is a schmitt trigger and, for reasonable input swings (you CAN modify the level with a couple of resistors) can be used, but DO remember that the Vil and Vih has very wide tolerances.

Or using a anlalog multiplexer to one comparator and use serial comunication. just exampels.
Many '51 derivatives have built-in comparators and/or built in A/D converters. e,g, the SILabs f12x/f13x has an 8 input A/D and 2 comparators. The sub $1 LPC '51 chips have comparators and A/D.
Maxim has (IIC/SPI/parallel) A/D converters with 8 inpus (check, maybe more than 8).

If using single chip comparators i need perhaps 100 000 / year Of cource you get a better price but the 0,5 USD is hard to beat. Well, Maxim has a quad for $0.49 (= $0.12 per comparator) @1000, probably about $0.20 @ 100k.

Erik