Exemple of what I have in my inventory will give you maybe an idea of the rate of some letter, in my exemple it will be about the A-B-G letters.
So Let's go :
A =0
B= 0
C = 32
D= 73
E= 69
F= 137
G= 1
H= 25
I= 56
J= 39
K= 131
L= 213
M= 3
N= 77
O= 117
P= 153
Q= 232
R= 303
So I have 1661 letters which represent 16610 tera thalers which represent for a rate of 50€ for 1000 tera Thalers = 830€ or 1007 dollards.
With 1000 dollards you dont have an A or a B.
So you know that a A or a B are less than 1/1661 chance to get by opening a tome.
But that's only my exemple and I am an adult who pay the tome with the money of my hard work, it does not mean that I am not entirely disgusted by that poor rate.