W Def and M def after Big Bang?

From what I can tell from fiels ever helpful work, the damage reduction by 1 point in wdef/mdef varies by how much attack the monster has, as well as how much defense you have. Generally the more defense you have, the less 1 point of extra wdef makes, since its proportional to the square root of your defense (unless you're considering one of the rarer cases where its a static reduction, at which point you'll have enough defense that survivablity will almost certainly be a non-issue)

Fiel has a calculator here as well as a list of monster attack values to use with it here
Not entirely sure about Mdef/Monster matt values, since not much has been said about them, but they're probably similar