I'm fairly sure Mighty Max batteries are manufactured by Universal Battery, which I believe is out of China. So I would compare their prices to the similar models by Universal, which you'll see widely around the internet.
I haven't used them, but my feeling about Universal and Mighty Max is that they're cheap and not premium but probably fine.
I think the difference between cheap batteries and expensive ones is lifespan. Unlike cheap tires, which can be really unsafe, I think most cheap batteries will function fine, but won't last as long as better quality ones. For AGM specifically, that comes from the thickness and quality of the lead plates, as well as the complexity of the design.