"Cheap" and "Amazonia" don't go together. But, when I researched this it looked like Aqua Forest in San Francisco has the lowest price. It would be worth the trip to SF for you, even with the cost of gasoline today.
Shipping from AFA is cheaper for me in Kentucky than shipping from ADG, because of their "free shipping" (not really free) model that ends up costing more in the long run.
When you compare Amazonia to other nutrient-rich substrates, active substrates or dirt/clay-based processed substrates, it's often cheaper. Yep, cheaper. For $30, you get 9L of substrate. Compared to UP Aqua or Azoo, you'd be paying anywhere from double to 6-7 times that.
If you live 50 miles from San Francisco, as the OP does, then AquaForest has a good price, compared to other local suppliers of Aquasoil. But, when you have to pay for shipping it, that's can be a different story.
ADG does not have free shipping per se, its just added into the cost. That being said, go select what you want on both websites, estimate shipping in the shopping cart and add it up. AFA is cheaper for my zip code.
But if you actually go to Aqua Forest, the price is higher than their website. I think a 9L bag of aqua soil was $38 compared to $30 online. I asked them about it and they said it was to offset the shipping cost. I usually get everything shipped since its cheaper for me especially with gas and bridge tolls.