Like Bobson, I didn't understand why doubles cost so much more than pumps. That is, until I became familiar with what a good double will do. Take my 101, for instance. Here is a firearm with two barrels and one trigger. Pull the trigger once, and one of the barrels fires. Pull it a second time without doing anything else and the other barrel fires. Move the trigger to over to the side and the barrels shoot in the opposite order. Open the action and the shells pop out. That is, only the fired shell(s) pop out! How does the gun know?!! You see what I'm getting at - the firearm is an intricate, complex machine made out of tiny parts that work every time in an amazing manner. On top of that is the engraving, the nice blued finish on the outside, the spot polishing on the inside, and some handsome wood used for the stock.
Wow! $800 is a bargain!
This is why a good O/U is expensive. I don't much care for the auto-ejectors and mine has an auto safety whenever I close the action. But ensuring that this action is reliable and doesn't break takes some craftsmanship.