Actually most aftermarket drums come with the inner seal ring pressed into the drum (example):You did not mention if you changed the inner seal and reinstalled the inner seal ring or ABS tone ring from the original drum....Aftermarket drums do not come with those parts installed .
Maybe some really cheap parts store stuff doesnt but if not you can always swap the ring from the old drum to the new.
The inner seal itself (on the spindle) rarely wears out or needs replacing. Just be certain its clean and has a bit of grease on it where is rides on the drum seal ring. And yes the replacement Ford part is ridiculously expensive.