I must have gotten lucky and picked up a Magnavore 4 ER from That Pet Place for $68. Looks like they are out of stock now though.

I also picked up the snap on brush piece and I'm really glad I did, it allow me to get along the sand line pretty well without getting sand between the pad and glass. Some sand gets stuck in the brush occasionally, but it doesn't have enough force to scratch the glass.
The other nice thing about this magnet is that the inside piece is bigger than the outside piece, so it allow you to get along the edges past the trim a little bit and gets into the corners well. I use the acrylic pad on my glass tank and it works great, plus it doesn't catch particles that can scratch like the glass pad does. After dealing with mag floats over the years, I'd gladly pay full price for one of these instead of going back. As far as coraline goes, you will always have to razor/scrape corners and hard to reach areas, but this does a good job of minimizing those spots. You should never try to remove coraline with a magnet scraper anyway, if it breaks off under the pad you will end up dragging it across the tank and scratch it.