So I'll go out on a limb and admit my ignorance. What is the main problem with dumping it down the drain? Exactly how toxic or hazardous is it? I've always treated it just like my spent developer and stop bath, not knowing any better. Of course, there is a lot less fixer to dispose of since I reuse it many times before disposing.
Recently I mixed up a 5 liter batch of Kodak fixer and used it for something like 4 months and 26 rolls of 120 film. I never used more than 500ml at a time and then just dumped it back into the 5 liter jug. I was amazed that it continued to work just fine. I only dumped it because I was leaving the U.S. to return to Japan. I guess there was probably a pretty good amount of silver in that jug by the time I was done.
So, how much do you get for the silver dust if you extract it? I suppose the first question is how many rolls do you have to develop to even pay for the silver magnet? Or is the primary reason for extracting silver environmental?