Of course no one knows, thats not the point
I'm thinking 70-80. Anything below that would require some crazy panic, and if it reached that just on "OMG price is dropping" based panic, then it would be completely unsustainable.
Right now, at ~$90, this is a "sustainable" drop, IMO. I cant see a big buy triggering us to go over $100 [unless the buy was massive], but I can imagine some big player taking a chunk of the 80s, and then sliding thru into the high 70s.
The whole reason people didnt sell when $90 fell is that there was no depth, and people were smart enough not to take less than they could for their coins. But there are many people lined up and ready to sell if the opportunity arises, I think.