Some of them were free! As a result of the previous screw-up where the assembly place destroyed 100 DB-19 connectors, before we knew they were so rare. Overall average cost was around $1. I located about 100 others that I didn't buy, because they were more like $5 each.

1990 might be a little off. I think the last Mac that had an external floppy port was the IIsi, or maybe the Classic II. Both were discontinued in 1993.