Okay, so looking at those listings on eBay.ca (for currency conversion to Canadian dollars):

https://www.ebay.ca/itm/IBM-FRU-81Y9731-1-TB-7-2K-RPM-SATA-2-5-HD-81Y9730-81Y3829-Lot-of-2-Matched-/254320589894?_trksid=p2349526.m4383.l10137.c10&nordt=true&rt=nc&orig_cvip=true

Just the bare drives, no caddy, and $90CAD for 2. That's not bad. No shipping cost for Canada, though.

https://www.ebay.ca/itm/Lot-of-12-IBM-81Y9730-IBM-81Y3829-1TB-SATA-6G-7-2K-SFF-HDD-81Y9731-/113822291903?_trksid=p2349526.m4383.l10137.c10&nordt=true&rt=nc&orig_cvip=true

Okay, so $800CAD for 12. That's $66.66CAD each. Not too far off what I'm asking, and I'd certainly accept lower for purchasing the lot.

And with 36 sold, this also meets your "ones that have actually sold": https://www.ebay.ca/itm/bulk-in-BAG-LENOVO-IBM-81Y9730-1TB-SATA-6G-7200RPM-2-5-in-HDD81Y9731/231969351413?hash=item36027366f5:g:fE0AAMXQC1hSL2od

You keep saying saying "$50 a pop" - $75CAD is currently $54.36USD, so my prices are in well within your ballpark figure.