$3 ADCs are not cheap ADCs. Cheap ADCs are the one embedded inside cheap MCU. These days, TI sell cheap ADC/DAC that is artificially limited to 10/12-bit, but with linearity at more than 16-bit. The cost of producing bad ones versus good ones are not much different. They go through the same semiconductor process. So long as the fundamental design is sound, cheap ADC don’t perform any worse.