Here's an idea I've had kicking around for a while. Go to your local dollar store and buy a cheap umbrella (not an automatic). Take off the handle, but leave the long neck tube. Slide the tube into a 1/2" or 3/4" PVC pipe, and connect like the diagrams show. Use the PVC Camera Mount (shown in blue) to connect the contraption to your camera (in dark grey), and there you have it.
The second design puts the weight of the camera directly over the monopod pole, but it also eliminates the ability to turn the camera sideways.
And of course, you can eliminate the pole altogether and use it for mobile shots.
Price: Under $5.00, if you can find a dollar store umbrella.
Build time: 10 minutes, assuming the handle comes off the umbrella.
Availability of materials: Readily available.
Durability: The PVC is durable; if you buy a cheap umbrella, I doubt it will last too long.
Functionality: Having never built one of these, I can only speculate that with any sort of wind, the umbrella may go flying (once I actually build one, I'll see if I can think of an easy way to secure it). Other than that, I can see how this could come in very handy.
Portability: About the same as a PVC monopod.