I've seen the opteka brand you were looking at online and read some reviews. I'm also looking for an inexpensive fisheye. Of course...with most things inexpensive....there is a quality tradeoff that is bound to happen.
If you are like me, then paying $400-900 for a quality fisheye that is used or new simply isn't feasible at the moment.
--I noticed that you had a link to the .35x opteka fisheye adaptor.
--You might like to consider the .20x opteka fisheye adaptor instead. Because of the smaller negative value of the magnification, you will get a wider angle of view that is more true to the effect of a REAL fisheye lens. If you throw this on a 18-55mm 'kit' lens, you can get different fisheye effects as you zoom in and out. At 18mm, you should get a nice full circle inside your frame. If you push it out to your 55mm setting or mounted it on a larger zoom lens, you should get a nice full frame image with the effect of the fisheye.
--Another 'middle-of-the-road' option is the Russian made Zenitar 16mm Fisheye. They will cost you about $200-$250, but are very high quality. Check out the review at the following: