"Cost" and "Price" are two entirely different things. Price is always based on the balance of supply and demand - those price hikes are largely due to demand being higher than supply. Cost, however, is much more predictable. Depreciation is the biggest factor for cost per wafer, but even as depreciation is being written off (typically in a straight line bases over useful life), as the process matures, yield increases (sometimes very significantly), so cost per KGD drops. However, as I noted above, if we take depreciation and yield improvements out of the equation (focus only on variable costs - same as marginal costs), I don't think those drop notably as the process matures.
Viewed from a different perspective, the costs for taking an IC into production are heavily weighted to fixed costs versus variable costs. Therefore, the goal for a company fabricating ICs is to keep capacity utilization high (divide fixed costs over the maximum number of wafers). That is why prices (and margins) decline sharply when we operate with excess capacity.