Ecomonic benefit of Castles?

[Carwyn]

I got in a discussion of castles with a friend. I find that I don't know how to figure how much they cost me and how much profit to get from my extra trade routes. I'm playing the Rise of Mankind mod if that matters. Any good formulas out there? Or threads on how that all works? I'm also really interested in how to develop a strong trade economy and any tips on making money.

Thanks!