Thanks for posting this. I am a so called RS2 product myself, and whenever I see screenies like those I get the feeling it's part of ancient history, though it's only a few years ago. :)
Judging by all the drop logs I have seen I would estimate the drop rate to be about 1 in 90. I got one on the 174th kill with my friend, we're at 200 now and still only 1. Good luck!