Cheapest per usable TB
Usable space
Cost of the drives
Every drive, ranked
How it works
Usable space is the drive size times the drives that hold data: all of them for single drives, one for a mirror, the count minus 1, 2 or 3 for RAID 5/6 and RAIDZ1/2/3, and half for RAID 10. Cost is the drive count times the lowest current eBay price for that model and size, so the cheapest drives may be used or refurbished: check the condition before you buy. Sizes are in decimal TB as drives are sold; your NAS shows about 9% less in TiB, plus filesystem overhead.
Pricing the rest of the box? The home server builder fills every slot, and all drives are ranked by $/TB.