Amazon offers the Transformers Trilogy on Blu-ray
(pictured) for $26.99
with free shipping
. That's the lowest total price we could find by $6. Sales tax is added where applicable. It includes all three films: Transformers, Transformers: Revenge of the Fallen, and Transformers: Dark of the Moon.
Of note, also available for Amazon Prime members is the Transformers Trilogy on DVD for $21.99 with free shipping. (Non-members can spend $25 or more to get free shipping; otherwise it adds $2.98.) That's the lowest total price we could find by $9.