Right now on Amazon you can pre-order Big Hero 6 (Blu-ray + DVD + Digital HD) for only $19.99.
And the cool part is that Amazon offers the lowest pre-order price guaranteed, which means if the price decreases between your order time and the end of the release date you will receive the lowest price.
I can not say enough good things about this movie. I had the chance to see a sneak preview and it was WONDERFUL- I have been telling people about it since.
Be sure to double check the price before you buy- Amazon prices can change quickly!