Walmart offers the Firefly 15th Anniversary Collector's Edition on Blu-ray for $19.99 with free shipping. (Amazon charges the same; Best Buy charges the same with in-store pickup.) That's the lowest price we could find by $9.

Note: This item is listed as a preorder, but was released on September 19.

Update: The price has increased at Walmart; however, Amazon and Best Buy still offer it for the same price.