Actually I believe 2-5 to be 1 in 666 and lvl 6 is "supposed to be" 1 in 250. This is due to the fact that you can't sell back the illusion cards for points and bosses only have a 5% chance to drop the 50 points tokens.
That's my understanding too.
I have to agree with the OP here - the drop rates appear to be grossly out of wack. It used to be that for every 90 mins (at the most) you would see a charm upgrade. I've been able to go 4 hours without seeing one. Could be incredibly bad luck - but this seems to be the case for the last couple of weeks at least.