Cost of PBL

Recently, on a blog that I follow, a case was made for PBL being less expensive than traditional schooling in a rural setting where an entire school converted to the PBL structure. This is very interesting to me, as my own district is considering closing a school due to declining enrollment (one K-5 school has under 60 students.)

If it is true, which I tend to doubt, I need to know the whys and the wherefores. Does anyone know of any research or analysis of this kind?

Thanks...