NOTE: By probability, I mean the chance of it happening per iteration, and starting with a new page each time.
Let's take a step back, shall we? (Not too many, because there's a cliff behind you.) Let's think of what the probability is of producing the following randomly:
A
Assuming there are only 26 characters (A-Z, uppercase), the probability would be $\frac{1}{26}$.
What about this:
AA
It'd be $(\frac{1}{26})^2$. This:
AAA
It'd be $(\frac{1}{26})^3$. And this:
XKCD
It'd be $(\frac{0}{26})^4$. [Just kidding, it's: $(\frac{1}{26})^4$].
So, for every character we add to the quote, it will be: $(1/26)^c)$, where $c$ represents the number of characters.
Basically, it would be a probability of $(\frac{1}{26*2+12})^c$ since the characters used could be: A-Z
, a-z
, .!?,;: "'/()
Of course, there could be more characters, but that's just an example. :)