Comparing industrial vs. off-the-shelf analog input
Generally it's not the ADC capability that you're paying for.  At the end of the day the actual ADC will be working the same way whether you have an Arduino or an expensive PLC.

What you are paying for is what is around the ADC:


Stable / filtered reference voltages
Low noise instrumentation amplification
(sometimes, depending on application) galvanic isolation
(sometimes, depending on application) environmental isolation
Longevity.
Standards.


Most of which you could theoretically achieve with an Arduino (or a Raspberry Pi if you added your own ADC system to it since it doesn't have one of its own), but by the time you have developed the system to give the same kind of performance and stability as the expensive PLC you'll probably find that the costs amount to considerably more than the PLCs.

And then when your system goes wrong, who do you turn to?   When your boss says "It's not working" you would then have to find out why and get it working again.  Or you can turn to Siemens and say "It's not working" and they can fix it for you.  If you have left the company, who is going to keep your system running for the company?

Many environments and industries have strict regulations around what equipment can be used.  Getting your equipment certified for use in, say, a petrochemical environment costs vast amounts of money.  Probably more than the total you have ever spent, or are likely to spend, on PLCs etc.