Don't think there's a guide to that stuff yet. Most 2nd level weapons cap at level 61, so what you can do is level the weapon up once, and look at the exp difference. If the current level needs 800 exp and the next needs 850, then each level will require 50 more exp. 900, 950, 1000, etc. So you can use that to calculate it. Someone needs to make a calculator for that stuff.