Ask HN: Turning a full-time job offer into a contracting proposal

Last week I had a reunion with the owner of a business looking to hire someone to refactor their application. I'm thinking about building an API between the modules and the db. This way the code gets cleaner, better distributed and new modules can be created in a more transparent way.

The problem is that this guy offered me a job but what I'd want is make him a contracting proposal, I've some other things going and getting a full-time job is not something I would think of. I'm trying to come up with a way to turn this into a contracting proposal, but I find it hard to make it because I don't know how bad and big is the codebase thus how much work is it going to demand. Maybe someone here with more experience can give me a hand.