Building an iso-matrix

I have a very basic problem that I cannot figure out. I have a (fairly simple) expression (attached) that I want to evaluate on two variables (within pre-determined ranges), k and sigma.
I’m trying to ascertain the domain of k/sigma indifference, i.e., the combinations of k and sigma lead to the same value for BSPut.

Thank you.

community question.nb (4.46 KB)