[External HDD] WD 18TB Elements ($470 - $155 = $315) [Amazon]

Really good price/TB!

If you don't mind a smidgen of finger-crossing, you could try the dude that sells refurbished data-center drives. I picked up a 14tb for a little over $130 or $140, and an enclosure with a fan from Amazon for $35. It's been great for plex, still chugging along after a year. Next step will be NAS with another 14tb in RAID