In addition, isn't the cost of a mask in it's design, not it's production?
They tend to be in the same ballpark.
There's a feedback effect: if the maskset gets cheaper you can afford to do shorter product cycles (think "iphone 15.5"), now you've got two masksets each with half the engineer-years devoted to it. This lets you be a bit bolder with the design since the cost of screwing up is lower. If masks get super-expensive then you only do one maskset and you throw more design-hours into making damn sure it's correct, and spend more time adding in "chicken bits" -- ways to turn off features that you screwed up, without ruining the whole chip -- and fallback options or insurance-policy type stuff. So the design cycle will lengthen or shorten if the engineering cost is several times larger or several times smaller than the mask cost. They sort of balance out.
Also the design cost is often impossible to define exactly. Blocks from previous designs get reused, so you can't really "charge" the effort it took to design them to just one chip. All the major design houses write a lot their own CAD tools too, and these tools have decade-or-longer lifetimes.