Amazon offers Prime members the Christopher Nolan: Director's Collection on Blu-ray for $18.49 with free shipping. (Non-members can spend $35 or more for free shipping; otherwise, it adds $4.98.) Outside of the mention below, that's the lowest total price we could find by $4, although we saw it for $6 less via pickup in January.
It includes Memento, Insomnia, Batman Begins, The Dark Knight, and Inception.
Of note, this is temporarily out of stock, but can be ordered at this price for delivery when available.
Don't want to wait? Buy.com via Rakuten Shipping
has it for $19.99
with free shipping