Add a 2nd Bathroom or not?

[Damon Cluck]

I currently have a 3/1, approximately 1300 square foot under contract.  The house has a second living area, that is a former garage converted to a family room.  I am looking at approximately 3500 to add a second bathroom making the house a 4/2.  My property manager says that the house would rent for $825-875 as is, but could go as high as $950-$1000 as a 4/2. So the difference is about $100 per month in cash flow.  At those numbers it will take about three years for the improvement to pay for itself.  Thoughts?