FWIW, I once ran across a page about radioactive FEDs. (The FED was a brand of Soviet, and post-Soviet Ukrainian, rangefinder camera. It was originally a Leica clone, but evolved independently over the years.) It's basically the same story described in this thread, but the page has some quantitative measurements of the radioactivity of the FED's lens, measured in a high school science lab. The level was about 40% of background radiation (so total dose was 140% of background). This was measured, if I read correctly, in direct contact with the lens's front element.

I don't know if the numbers would be similar for the Canon lens in the referenced eBay auction, of course.