More cost efficiency (this feels like code golf):
- only 1 control line
- only 1 transistor (N JFET or P MOSFET), 1 R and 1 C
- infinite number of possible measurement ranges
- can get rid of the ADC, if it isn't needed otherwise (see below)
- needs calibration and could be temperature dependent, but the same goes for the switched R method at a lesser extent (so it depends on your desired level of accuracy)
- 1kS/s is ambituous with an MCU (needs fast control clock)
How does it work:
By controlling the duty cycle of control, you vary the DC gate voltage and therefore tune the resistance of the FET in a continuous fashion. You can also use feedback to bring AtoD to some defined voltage (e.g. VCC/2) and then your Duty cycle gives you the value of R1 (via a conversion which you have to establish once).
Down the road, you could even scrub the ADC (save costs) and realize the feedback with a comparator, so your MCU can read the duty cycle directly.