Modifiers, in general, are a way to add fees to a specific image that you are selling.  In the simple example, you can add retouching to an image for $5.  In a more complex example, modifiers can increase the cost for selling something based on size (like a canvas).  You can use them in any way you like.  Most of you will use the simpler example.  But for those of you that love math and want to take the advanced route, the following is just a jumping point. 

Here is how the "functions" work:

You can use price modifiers anywhere it says "(either a fractional number without currency or a function of area A/P)". basically, there are 2 "variables" you can use to create equations.

A - The square area of the print specified in Pricing > Prints > (set) > (New/Edit) > Print Width and Print Height

P - The price of the print specified in Pricing > Prints > (set) > (New/Edit) > Print Price

With these 2 variables, you can construct very simple functions or very complex functions.  A simple example would be something like:

A*1.1 - This would calculate as the area of the print times 1.1 (or a 10% increase)

That said, you can use any of the ECMA Math class functions to create more complex scenarios:

max(10, A*1)  -  returns either $10 or the square area times $1 -- whichever is greater

you can mix and match, obviously, and you could, if you were so inclined, to create very complex rules for pricing.

max(min(p*1.01, 100), A*1) - returns the greater value of the square area times $1 or the smaller value of the price plus 10% or $100.

With these examples, hopefully you can get started on creating rules about your prices.  Feedback is appreciated!