Sampling many low voltage analog signals
Source impedance is about 1 Mohm. ... Single-ended signals (common reference voltage). 4 bits of precision should be enough. I can correct for bias at a later stage, so any accuracy is fine.


This likely means that you will need to buffer your sources somehow. Mainly because of whatever switching scheme you use, each switching event will require charging at least some capacitance before the signal at the ADC settles.

Since the signals are single-ended, you can use an op-amp rather than an in-amp. 

Because of the low input signal level, you may want an op-amp categorized as a "precision" part, but you don't need the best of the best here. Depending on thermal conditions where you will use this circuit, you might get away with a general-purpose part.

I see quad general-purpose op-amps for as low as $0.11 (qty 1000) and precision op-amps with Vos below 1 mV as low as $0.45 (qty 1000) at TI. Figure $0.50 to $1.00 each at your quantities (probably lower, really), that gives about $60-$100 for op-amps to buffer 128 signals.

I also see 16:1 analog muxes (such as CD74HC4067) for as low as $0.24 (qty 1000), again, figure a higher figure ($0.75?) for your low volumes. So maybe $6 for muxes.

Then you only need 8 ADCs, each capable of about 16k samples per second, but with very relaxed precision requirements. These parts shouldn't run more than $1 or $2 each. So maybe $16 for ADCs.

Without knowing a lot more about your problem (and spending a lot more time thinking about it) I can't tell you specific op-amp, mux, or ADC part numbers. And you'll also have to do some more engineering to synchronize the control signals the way you want. But I think you'll be able to keep this under $200 without too much trouble.

Edit: I thought more about your op-amp requirement and realized you probably won't want to just use a $0.11 part.