New drives are around $16-$20/TB depending on if you catch a sale and are willing to shuck. You can pick up used SAS drives in 3-4TB capacity for around $5/TB. I'm a crazy person, so I built this to hold 11 3TB SAS drives:
https://imgur.com/a/JqbBN1p
https://pcpartpicker.com/list/dLhNvf
I used a Supermicro MB and ECC RAM. It's not much more expensive and it's nice having IPMI. I personally think it's crazy to forego ECC. The SAS controller, expander, and drives were used, everything else was new. Prices have gone up. The new parts were $638 at the time. The drives were ~$20/ea. The HBA and expander were ~$85 for both. After the fans, cables, extra drive cage and brackets, total cost was around $1K. This hardware is all supported out-of-the-box by TrueNAS. I haven't done the math to figure out when the cost of running this will exceed having purchased higher capacity drives.
This is what a typical used SAS drive looks like. 30K hours hours but very few on/off cycles. Zero defect list:
https://pastebin.com/WcUYX4JR
The failed SMART test turned out to be a firmware issue. I had to update the firmware on all the drives. That was a bit of an adventure:
https://pastebin.com/j3AGX0xN
A few drives arrived with a non-zero defect list or otherwise failed burn-in. I contacted the seller on eBay and they sent me replacements w/o any fuss. I'm not necessarily recommending used SAS drives, but I'm not recommended against them either. I will recommend the serverbuilds forum for generally good advice on all this. I think this post got me started:
https://forums.serverbuilds.net/t/another-nas-killer-4-0-bui...
The current NAS killer version is 5.0:
https://forums.serverbuilds.net/c/builds/18