Walmart offers the Seagate 4TB Backup Plus USB 3.0 External Hard Drive, model no. STCA4000100, for $129.99 with free shipping. (Amazon charges the same.) That's $0.03/GB and the lowest total price we could find by $20, although we saw it for $10 less last week (since expired). It features two backup scheduling options and social network sharing.

Update: Headline price corrected.