What is the least expensive and decent full face powered respirator (i.e.
dust mask with a fan to draw air)?

The reason I want a PAPR is because wearing an unpowered dust mask makes my
face hot and sweaty. It also fogs up my goggle if I am wearing one.

The one I found most commonly listed in "froogle" is a trend airshield that
sells for $300. Isn't there something less expensive and works?

Or perhaps someone has a plan to make one? Surely the materials don't add up
to $50 (air filter, some plastic shield, a fan).