What is the cheapest (yet correct) amount to add a bathroom?

[Michael Johnson]

Hey BPers, quick question. I found a 4bdr 1ba. So I would go ahead and call that functionally obsolescent?? Especially if I want to rent out each room individually. I know there are 1,001 variations but I have no idea how much it would cost. So im just looking for really rough numbers or a range. 

If it makes it easier dont include the vanity and tub. Im looking for the plumbing, construction, and permits I guess. 

Thanks!