The cheapest 8TB HDD I could manage to find on eBay that isn't refurbished or OEM.
Normally $239 from other computer stores.
You can also buy it for even cheaper from Amazon USA for $226.44 (Amazon Prime required for free deliveries)
and if you know how to "Shuck" a drive (ripping a HDD out of it's external enclosure) you could save even more by purchase an 8TB Desktop drive for $208.11 from Amazon UK (Prime required for free delivery). These are likely BarraCuda SMR drives.
Just beware the lack of warranty if you do so and hope that the courier doesn't drop your parcel.
As per this Seagate CMR & SMR list, these are SMR drives