Product review (5 stars): Great Range for the Price
I paid about $150 plus a little more for a circular damper.  Lots of air is moved.  There are some cons like the finished look on the outside.  You can see the seams so it does not look sleek.  There are some air voids that allow the fan to blow some air back into the kitchen as opposed to pushing 100% of the air outside.  Easy to install (about 2 hours including duct, roof vent, and cutting holes in cabinets).