You don't need a masters degree in math to teach freshman calculus.
I was taught freshman calc by grad students - with no "education" training whatsoever. Perhaps these "masters" of education are learning the wrong things.
Freshman calculus just isn't that hard, and isn't much beyond algebra. This "gee math is hard" crap is, well, crap. No PhD is necessary. Hell, I taught it to my kids on the kitchen table, and I have no advanced degree and never took a course in education.
Do you think someone needs any education at all in a subject to each it? Do you think someone who has never studied calculus should be able to teach it?
Right, well here's the answer to the original question then. If you want teachers to be able to teach calculus, and you agree people should have an education in a subject to teach it, then you need the teachers to study calculus themselves, which they weren't necessarily doing, because they studied things other than maths, so you need to use more resources to have them also take calculus classes, so it costs more.
Apparently school districts pay teachers more who get a masters. But it seems that having a masters doesn't mean they are able to teach a single course beyond high school level.
Sounds like things are all messed up in what districts pay for. They don't need more money, they need to redirect what they're already paying for.
The normally accepted education level for teaching a university-level class is a PhD.