If one is looking for adding a 1 TB WD Black mechanical drive w/ 7200 RPM + 64 MB cache (these are what I've used for years)...
Fry's has it for $50 w/ whatever the promo coupon code in your e-mail is, if you're on their newsletter/e-mail listing.
Only one per household on these allowed.
SHIPS FREE is you want it shipped to you or you can get it in the store.
Also if your looking for even more space microcenter has some deals on larger ones
wd 1TB 7200rpm - 40$
toshiba 5tb - 150$ (also have an external for 130)
4tb for 120, etc etc, heres the full list, various brands and sizes so you can usually find whatever your favorite company of the month is +size. The list also has all their ram deals, and since ddr3 is at an all time low its a good time to upgrade if you need more