I've recently tried a magic filter and a red filter from Aditech for my 10 Bar housing. I don't know if Aditech make the filter themselves or not, but I found that it was significantly better than the magic filter I used. I found that the magic filter took away too much blue. It seems all cameras are different however, and others have had great results with magic filters. I'm using a Panasonic GH2 with the 14-42 PZ lens (for video), and the only way of using the magic filter with this setup is to tape it to the rear of the lens, like they show you on the website.
I posted in the video section recently a clip from the Maldives (under GH2 hints, or something like that). I'm fairly new to this setup so it's not as still in my hands yet as I would like, but I think the colours have come out fine, at least with the WB technique I described on my vimeo page. Judge for yourself:
According to this:
the price for the Mangrove CF-M67 is 46 euros.