My fiancé and I purchased a home together and because of my credit we had to put the deed in his name. Now he wants to add my name to the deed. How do we do that? And how much will it cost?

At the top of the Real Estate Law forum there is a sticky post that explains how to add someone to a deed. You can read it at https://www.askmehelpdesk.com/real-estate-law/how-add-someone-your-deed-91136.html

I will add that you will also need to discuss this with the mortgage company since changing the deed sometimes is in violation of the mortgage rules.

I guess I still wonder why if he has good creidt, why the two of you could not still have gotten a mortgage, he would have still had good, that would not change