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What's the difference (electrically) between AC- and DC-coupled sampling circuits?

I've noticed that in analog signal acquisition hardware you see two paradigms: AC-coupled and DC-coupled. Typically AC-coupled hardware is less expensive, and basically has a 20 Hz high-pass filter to remove the DC component. DC-coupled hardware, which is good for reading sensor signals for example, tends to be in the hundreds or thousands of dollars, whereas AC-coupled hardware like computer sound cards is very inexpensive.

What are the electrical challenges in designing high-quality circuits for sampling DC-frequency analog signals? I'm trying to understand why there is this huge difference in cost.