I planned on buying one of these, but I would rather spend the money on tools that directly impact the quality of my projects. I built this using an attic fan that I was given, some left over wood and a scrap piece of extension cord. I purchased the furnace filters (there are 3 of them in different MERV ratings), the hinges, the clasp, the electrical switch and box.
The only thing i need to do is build the blade guard on the exhaust side, add some eyes for hanging and then figure out where to hang it.
I have run it for several hours and WOW—these things make a huge difference in air quality.