I'm doing a master's in math. My undergrad was math and econ, and I'm staying at my undergrad institution for the master's. I think the value added will be quite a lot, actually. My thesis is econ focused, but math based. That can't hurt. I'm funded. Also can't hurt.
"Prerequisites" for my program were pretty simple : some proofs based course. Some linear algebra. Some other algebra. Calculus through multivar. Many students get master's in math that didn't do it for undergrad.
I would warn, however, that it might not be a good idea unless you actually like math
