There were high quality versions of these which were the Raynox and the others made by Olympus and Sony.
These were however more useful with the bridge cameras or as they would call it superzooms like the Canon S3 IS and the like (G7, G9's).
These where attached to these bridge cameras to provide some sort of an SLR/DSLR psuedo-interchangeable lens feel since these superzooms had fixed zoom lenses and the only way to alter them was with these additional screw on lenses.
These high quality lenses however, costwise, are in the range of real SLR/DSLR lenses.
You might want to visit lensmate if you want to know more about them:
Canon PowerShot G9/G7 Lensmate Products and Accessories
I had a Canon S3 IS before and this is how I learned about them.
It would be best however to stick with SLR/DSLR lenses with the same costs.
Those $20 are most probably crap.
If you attach these on the wrong lenses also, some vignetting will result (for wide angle).