I have been collecting ink and laser cartridges for a little while now. As far as I know there is no metal value to them, although I have never taken them apart, could be a little copper?? Not sure.
Staples, and other stores offer this credit in order to get the empties and have them refilled, or re-manufactured to sell again as an off brand, cheaper cartridge.
There is another thread or two on here, but not a whole lot of information in them.
I suggest selling them as empties there, are some websites, toner buyer.com( thread on them as well) or you could find a local refill shop and see if they maybe interested in them. I have a store owner calling me today to see about selling her a hundred or so.
What types do you have and where are you located?