What’s the marginal value increase as toilet price increases?
There are different materials, i.e. porcelain (vitreous china) or ceramic. They are similar, but there are cost differences based on the manufacturing process. Porcelain is baked at higher temperatures, so they take longer to cool, resulting in fewer toilets per day from the manufacturing plant. Ceramic however needs additional coatings to make it smooth, but people are wanting things like anti-bacterial coatings, oil resistance etc.

One piece toilets increase the price too because if something cracks or splits in the baking / curing process, they have to throw away the entire thing, vs only the tank or only the bowl. But when you have separate parts, you have more nooks and crannies to clean.

After those main details, it mostly becomes marketing, aesthetics and hype, often about that the toilet is capable of "passing". I bought one that famously claims it can "pass a tennis ball", that was total BS...