What's the difference (electrically) between AC- and DC-coupled sampling circuits?
Components are always a big cost. When you need high accuracy resistors, you can run up your BOM cost quickly. Add in some pricey op amps (which are also my favorite, unfortunately), and you can have a 100 dollar board in no time. If you want to match the high speed A/D technology of the AC coupled circuits you speak of and want to do it at a high resolution you also will be paying through the nose. Just the other day I was looking at an 18 bit, 1 MSPS A/D that topped 25 dollars. You could build a lot of cool stuff with that same 25 bucks.

Also, a key thing to remember is 1/f noise. As you go towards (absolute) DC noise goes through the roof. DC is a tough field, but I work with it everyday so if you have any other questions, give me a shout.