Draw a curve with defined variables

Hi everyone,

I warn you already, I'm french and there might be some english math vocabulary I don't know, so I'll try and be as clear as possible in my explanation, and call each variable something.

I work for a construction insurance company and I need to draw a curve either on Excel 2007 or Grapher on Mac OS X (preferably on excel) that would represent the premium for constructions with an amount from 10,000 € till 1,000,000.00 € (with a step of 10,000 between each points, so a total of 100 points: e.g.: what's the premium for a 10,000 € construction, then for a 20,000 € one, then 30,000 €, etc). Let's call the total construction amount "x", and the premium for it "y" (or f(x))

For each 100,000, an intermediate premium "p" is set (not proportionally).

For instance:

from 100,001€ till 200,000€, the intermediate premium is 10,200;

from 200,001€ till 300,000€, the intermediate premium is 10,400;

from 300,001€ till 400,000€, the intermediate premium is 10,500;

from 400,001€ till 450,000€, the intermediate premium is 11,800;

from 450,001€ till 500,000€, the intermediate premium is 12,500;

The premium is calculated with the sum of many percentages, and percentages on THESE percentages... But basically, it's the sum of 3 main objects:

A: the premium before commissions

B: the company's commissions

C: the broker's commission

Is there a way to define many variables to begin with, and then name stages of the equation for later use to simplify things (for instance, the intermediate premium "p" + 5% becomes "P", so then when i need to use it to add or multiply with another percentage i can just use "P")? Or maybe there is a sort of math terms or odd greek sign (dont know if matrixes are of any use here) that could simplify things a lot (Grapher is quite a complete program so i could use all these terms in it)?

As you can see, this is actually more about how to use either Grapher or Excel 2007.

If you need more details or infos, I'll check the forum daily.

Thanks in advance to whoever can help (Nod)