A 4tb drive costs 80-120€
7€/month for 5y
that price is for barracudas. then you will be lucky to replace them every 2 years.
wd gold and others that will last 5~10years are actually more expensive for 4TB ($500) because they are old stock for people that for some reason need that model.
The sweet spot for top quality HDD is always 200~300. In whatever capacity du jour. (14~20TB today)
nitipicking drive cost aside, the monthly services are the bulky of the cost.
zfs rent = 4+TB drive + $10/mo (base 1TB) + $5*3 (extra 3TB) = drive + $25/mo
hetzner = 5TB $10.9/mo
vultr = $5/mo (probably more)
4x ~4TB drives = $800
assuming you replace drives every 5yrs: $3,254 or $54.23/mo.
assuming the drives last 10yr which are common now: $5,708 or $47.56/mo.