Here's a typical calculation: A home darkroom processes, say, 2000 sheets of 8x10 black and white photographic paper a year. If half the silver goes down the drain that's 3 ounces. If, instead of disposal, much of that is recovered and sold the net return after refining costs could be up to $30 per annum on today's silver market. A lot of people would say its not worth the trouble. Opinions differ.
If you choose check with local authorities, ask their permission for what you intend to do, and they say no, I guess you have to comply.
your calculation is a little off ..
there is about 1/4 oz of silver in a gallon of well spent fixer.
i am not sure how much film or paper that is in a year, or how often you
use a gallon of fixer, but its about 1/4 oz/gallon ....