Karma cost equations, Math people needed Options

[PlatonicPimp]

OK, so it has become apparent to me that I've lost over half the math Knowledge I once had from schooling through disuse.

I am trying to render the Karma cost of various improvements in terms of a mathmatical equation. We all know that raising, say, a skill group costs (5 x New rating) for an increase of 1.
How would one render the equations that shows the growth in cost per die over increases of more than 1?
My desire is to write how to figure the Karma cost of any skill, skill group or attribute increase as equations, allowing you to plug in X (the rating you have it at) and Y( the rating you want it at) and find out Z (the Karma cost to increase to that level.)