[slitz]

Yeah that's one way to do it, but it wasn't what I was talking about.
Basically, you've made the formula itself static while keeping the variables changable. What I'm talking about is making the formula itself changable.

Lets use a very simple example:

public double getPrice(double rarity, int durability, Material material){
             return (rarity*durability)/material.getPrice(); 
}

Now what I've done here is hardcoded the formula. No matter what I do the price will always be rarity*durability/material.getPrice();
In some cases this will never ever change, but in other cases it might! (combat formulas etc)

public double getPrice(Formula formula, Item item){
          // parsing logics here which apply formula with item variables
}

Now all of a sudden I can change the formula of a item at runtime, as long as I have a functioning evalutor (which isn't hard to do). Meaning, I can even read the formula from a XML file / database.
Note this is rather complex way of doing it if you ask me, so it shouldn't be used if you only need one formula for combat/item values/item droprate/whatever, but if you're looking for a bigger project I think that it sounds like a good idea in theory at least 8).