If you keep an eye out for sales, you can get new drives for not much more than used. I got two Seagate Exos X20 20TB drives for around US$240 each on sale. One from Newegg and one from ServerPartDeals.
Regardless of if you buy new or used, buy the drives from multiple different suppliers as it makes it likely that they'll come from two different batches. You don't want an array where all drives came from the same batch since it increases risk (if there was a manufacturing issue with that batch, it's possible all drives will fail in the same way)