Happy to answer that question! In the end it's a numbers game. Manufacturing gets cheaper with higher demand. For 6 layers the demand is rather low. We've figures from a US PCB comparison site and the share of 6 layers and more is below 5%, everything else is 2 and 4 layers. JLC serves a huge domestic market, so even with niche products they can do order pooling. That's not possible for us, thus the difference between 4 layers (€28.29) and 6 layers (€184.83). If there would be more demand in general, we could offer much lower prices.