If you go with 12TB drives, these are available, with a 2-year warranty, for $74.99 USD. That’s $6.25 per TB.
The above is the lowest cost per TB on their site, and the five-drive total (less shipping and taxes) is $374.95.
If you are not committed to a RAID-Z1 solution, you could mirror two 16TB drives at a cost of $129.99, each, for a total of $259.98, from here (also a 2-year warranty):
The cost per TB would be higher, and the performance of a traditional RAID 1 will be less than a RAID-Z1. But it would cost $114.97 less.
Both are the same company that ships from Florida, USA.
They are reputable, and easy to contact.
But the overseas shipping costs might be prohibitive.
I live in New Jersey, USA, and they ship for free to my destination.
eBay is another option, if you can find enough low-priced drives+shipping for your location. Or someone selling their farm, local to you, might chime in with an offer.
Just thought that I would present the Florida company’s offerings, as it would be hassle-free, with a good base price, and your RAID would consist of the same model drives (if that matters).
They have drives with other capacities. But I focused on the above based on what you are seeking