Oh of my main "guys" in the Czech Republic regularly takes extra cases to the smelter in exchange for cash. I can't remember the specifics, but a 20-micron case in good condition had on the upper end of one solid gram of gold (!) and netted something like $20 (today's prices). And that's for *one case*. That can be a seriously lucrative practice, especially in areas where economies are depressed or where local currencies are unstable.I would love if anyone with some personal experience in this could chime in. I could be totally wrong about all of this, and it would be great to get a first- or at least second-hand perspective.
Of course, as you mention, 10-micron cases are half that, and many cases have at least some of the gold worn away by now (so could be even less). So you'd need a few decent early cases, or a lot of crappy older cases, to make it a real "business". But the payout is still much higher than what you calculated, so I'm not sure how all the math works out.
Thinking from a seller's perspective, the practice of case harvesting can be very attractive. Rather than dealing with listing on eBay, writing a description, taking/uploading photos, waiting for a buyer that may never come, dealing with shipping, responding to incessant buyer complaints, returns, lost packages, eBay taxes, etc., it'd be sooooo much easier just take your cases down to the smelter and get cold hard cash, in hand, on the spot. While I shudder to imagine millions upon millions of watches being melted down for scrap gold, I can certainly understand why.