Walmart offers I, Frankenstein on 3D Blu-ray / Blu-ray / DVD / Digital HD for $5.97. (Amazon charges the same for Prime members with free shipping.) Choose in-store pickup to avoid the $4.97 shipping fee. That's the lowest price we could find by $2.

Update: The price at Walmart has increased; however, the price at Amazon is still $5.97 with free shipping for Prime members.