How much value does a 3rd BR add??

[Chris McKinley]

I am currently looking at a house that is listed as a 2br/1ba. The previous owner finished the entire attic to make it into what they call a man cave/bonus room. It looks to have a lot of potential to turn into a 3rd br with the addition of a bathroom. I remember @brandonturner saying in his book that one of his favorite things to do was turn a 2br into a 3br. I'm sure it is dependent on area and some other factors but I'm wondering how much value I can add by putting in that bathroom? Thanks in advance for the help. Cheers -Chris