Dynamically populate product pricing based on prior entry on form

Here is the scenario.

I sell services based on square footage. I want the customer to only need to enter their [gross square footage] one time.

All services we sell are calculated from the [gross square footage] selection. The problem I have is that all of the services are priced diferently.

Can I create a drop down box that allows a customer to select their [gross square footage] (which I've done with no problem) and then create each separate product (service) field (I am assuming another drop down box) that are then dynamically populated based upon the original choice of square footage (I don't know how to do this --tutorial anywhere?) but calculate pricing differences listed in the "values" choice associated with each products drop down?

So for instance, my customer initially selects the services he would like from a checkbox group:
Services Desired:
x-Painting x-Prepping o-Cleaning 0-Staining

The product choices that are made above then display (via conditional logic):
[gross square feet-dropdown]
[painting-dropdown]
[prepping-dropdown]
has a surface of 3000 square feet, here is my proposed idea (customer selected variables are starred):

[Gross Square ft dropdown:]
<1000>
<2000>
*<3000>

Product 1 dropdown-
[Prepping Surface: ]
<1000> $1
<2000> $2
*<3000> $3

Product 2-
[Painting Surface:]
<1000> $4
<2000> $5
*<3000> $6
So then the Customer sees
Prep-$3
Painting-$6
Total- $9

So my goal is that [product 1] [product 2]get populated dynamically with correct price based on passing some parameter [that I don't know how to do] from the [Gross Sq Ft" dropdown] * [product 1 - value], same thing for every other service they choose.

But the customer only had to select square footage one time.

So.....teach me! How do I pass a value from within the same form? Example(s)?