There are several important formulae in this game. Here is a partial list of them.
Inn Price: (.25)H + M
H: HP lost
M: MP lost
Weapon Skill Points gained: U-L+X
U is the number of times the weapon is used
L is the level of the weapon
X is a pre-determined constant for a set of monsters [See bestiary]
Evade Rate: Agl + (R)(x) + (L)(y) - A
R is the evade rate of the right hand weapon
L is the evade rate of the left hand weapon
x is the skill level of the right hand weapon
y is the skill level of the left hand weapon.
A is the sum of the evade rates of the armor the character has.
Hit% = (Power+80) + W
W is the raw weapon hit % (I've yet to get all of this data, but I'm pretty sure this formula is right).