My son is in a math PhD program so have been living the math life for some years.
First, many if not most kids who think they are going to major in math change their mind once they start college and get exposed to upper level math material which is nothing like anything they have seen in their life. So unless she already has taken proof based math classes and loved them, do not assume she will graduate as a math major.
Second, future plans with math may make a difference. There are many kids at top PhD programs who came from public U’s. So if she shines in math at OSU there will be no difference in terms of future paths open to her. Also, OSU will have lots of graduate math classes she can take as an UG (I’m assuming there are no restrictions on this at OSU) which she will need if she is headed to math PhD. If not grad school, then you could check into the placement results of the grads from each school in whatever direction she thinks she will want to go in.
I agree that ND math is not worth 4x OSU math. The real question, as noted above, is whether ND is worth 4x OSU as a school to your family. We are paying paying 4x our state school cost for DD to attend a private U. Worth it to us but many people think that’s crazy.