However, if x86 has neither a process or a compatibility advantage, then even a tiny technical disadvantage turns into a tiny extra cost, which can be important for high volume chips.
Not true.
The marginal cost of pretty much anything on a chip itself is close to zero. For example, most 3 core chips are actually 4 core chips with one disabled. Putting the extra silicon on the chip is effectively free.
The money goes in the investment in the factory and the R&D, NOT the raw materials or production costs.