Target offers Deadwood: The Complete Series on Blu-ray Disc for $55.99 with free shipping. (Amazon has it for the same price.) That's the lowest total price we could find by $9, although we saw it for $6 less this past winter. This set includes the episodes from all three seasons and bonus content.
Update: Note that Amazon is temporarily out of stock, however it can still be ordered at this price for delivery when available.