In my do far short career I stumbled across a huge pile bad-written piece of code. So I want to estimate how long and how much it takes to maintain it and how long and how much it takes to just write from the start.

I want to convince my manager (and aid him) towards a direction that we should gradually re-implement pieces of our business logic. For that I need to show something in a matter of money and hours.

Do you have any methodology on how to do that?