I guessed right - I said $44K or $45K. It's $45K to unlock my latest book:
It's at $38K and change as I write this, and 1,687 backers.
So another $7K - it'll take as few as ~70 people at $99, or as many as ~2300 at $3.
I'd actually enjoy it more if it was 2300 additional backers getting my PDF, but that seems a bit unlikely. Something in between would do.
It is a relief to finally see my PDF on the list of potential unlocks. So please spread the word. There are almost certainly some people you know that would love to back this for $3 or more who just haven't heard about it.