what is good price for bathroom addition?

I just purchased a 4 bedroom 1 bathroom house that I want to rehab. I want to add a 2nd bathroom either upstairs or the basement. The problem with the upstairs is that the roof is slanted so there may not be much room for a bathroom addition without punching out the ceiling.

http://pics.rapmls.com/gstl/bigphoto/016/538016_10.jpg

Would it be better to just add the 2nd bathroom upstairs or in the basement.

Does anyone know what a good price should be for the addition? What is the best way to find a good cotractor? Home depot?