An asian 21j (either high or low beat) is theoretically a solid workhorse movement, but in reality it is subject to non-existent quality control. Yours will either serve you well for years or need replacing within a few months .. randomly. A base A21j (2813 or 4813) supports hours, minutes, seconds, and the date and will cost somewhere around 20 or 30 USD retail in single quantities. The main stumbling block here is QC. It simply doesn't exist. You get what you get and maybe it's broken .. maybe it's not.
An ETA 7750 is (first of all) a chronometer movement. In addition to it's watch function, it's a precision stopwatch, with multiple hands and sub-dials. It has 200 (or more?) precision finished parts, all whirring around inside of it. These movements are polished, decorated, calibrated in multiple positions, etc etc .. a part meant to be as perfect as possible out of the box. You are paying for it's complexity, the labor involved in assuring it's correct and of course the salaries of the workers who do all this .. probably 100x what a chinese worker makes.
By coincidence, you chose two movements which are completely at the opposite ends of the watch spectrum .. the humble chinese workhorse and the rarefied swiss super-stopwatch.
If we consider two movements more alike, like an ETA 2824 and a Chinese exact copy 2824, the retail cost difference is about 60 USD. The chinese clone ETA movement has less fine finish on the gears, no QC (again) and casual assembly practices.
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