Exactly. The BOM cost of a NAS with 3 2TB needs to be under $100. Even if we could somehow have 2TB 2.5" HDD for $30 BOM, three of them would have been $90.
How did you arrive at that target price? I find the Synology units to be a steal for what they offer. Its very hard to undercut this with a DIY build, best you could do is plug a USB drive into the wifi router I guess.