Costs involved with splitting a large bedroom

[Gary Dezoysa]

A couple years back I remember walking around a house with an experienced real estate investor in my area. He mentioned that in that part of town, three bedrooms rented for a lot more than two bedrooms, so he added a wall and door to separate one bedroom into two small ones. 

I'm guessing this is fairly expensive undertaking; can anyone give me a ballpark idea what the costs would be  for doing this in their area?