> An 8TB 3.5-inch HDD from Synology's Plus line, the HAT3310, costs $210 on Synology's web store. One of the original drives the HAT3310 is reportedly sourced from, the Toshiba N300, can be found for $173 at more than one vendor.

That's $37 of pure profit, per drive - and you'll usually have at least 2 drives. Synology's NASes start about $260 without drives. I'd wager they'll make more profit on the drives than they do on the NAS.