Amazon offers Prime members Mr. & Mrs. Smith on Blu-ray / Digital HD for $4.50 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's the lowest total price we could find by a buck. (Note: It may take an extra one to two days to ship.)
Also available is American Psycho on Blu-ray for $5 with free shipping for Prime members. (Walmart charges the same via in-store pickup.) That's also the best deal we could find by a buck.