I installed a half door to isolate our dog from the laminate flooring when he sleeps at night.
It was only $25 for the hollow core 6 panel door from HD, which got cut in half.
$25 for the passage knob.
$10 for top plate and trim
Less than $5 for the hinges.
Not having to replace the laminate flooring priceless!
The $25 door is not prehung. Buy the $14 dollar irwin door jig to install the door "http://www.homedepot.ca/product/door-lock-installation-kit-carbon/904157" It will come in handy if you are remodeling and installing new doors to replace those old flat panel doors. You will also have to chisel out the hinges.
When installing and marking the hinge locations in the door frame shim the door a 1/4" off the floor.