Best Buy via eBay offers the OEM Western Digital Red 3TB 3.5" NAS SATA 6Gb/s Internal Hard Drive, model no. WD30EFRX, for $104.99. Apply coupon code "C15FATHERSDAY" and checkout with PayPal to cut it to $89.99. With free shipping, that's the lowest total price we've seen. (It's a current low by $27.) This 5400 rpm hard drive features a 64MB cache. Coupon expires June 9.
Update: Specs corrected.