WD external hard drives are currently on sale on Best Buy and Amazon for $299.99. It might not be the best in terms of $/TB but it’s not too bad given if you’re running out of bays.
I also saw an email from Newegg for Synology DS920+ selling for $439.
Edit: Looks like the deal from Amazon is from Amazon UK so it might be better to just get it from Best Buy
[Amazon 18TB Elements](https://www.amazon.com/18TB-Elements-Desktop-External-Drive/dp/B08KY32HFR)