Variable Pricing Multiplier Option Instead of Fixed Price

I am using GF for a print shop site and the scenario I am facing is that the type of material the customer chooses (i.e. glossy paper or satin paper) significantly impacts the total price. However, I am only given - as far as I know - a fixed price option, which doesn't work since the size of the print also impacts the final price. Currently, the customer inputs the WIDTH and HEIGHT of their print, which gives me the sub-total but from there, I would like to use that WIDTH and HEIGHT sub-total to come up with a final price based on the material choice (set at a price multiplier).

For instance, if the customer selects the more expensive Satin Paper, I need for the price calculation to be a multiplier (not a fixed added cost) since the price of Satin costs much more than regular glossy. I see the +/- pricing options but that doesn't help me much because I can only put in a fixed price. Someone ordering a small poster that is 2x2" shouldn't have to pay the same fixed cost as someone ordering a 10x20' sign.

Also, I was looking at the PRODUCT field and setting it to CALCULATOR but this doesn't quite do what I am looking for.

THANKS FOR ANY/ALL HELP! All solutions and suggestions welcome!

Hi Chris, thanks for your reply. Is there a way to HIDE the customer from seeing the product field that I am using for the calculations? I may be able to use it (will give it a shot today) but can't get it to hide when choosing calculations. Thanks!

Would you guys mind taking a look at my form XML to see how I could get the pricing to be calculated properly based on the Material Selection and Width and Height?

XML: http://goo.gl/hf0Gz

Right now, the Material drop down only allows me to ADD a dollar amount (i.e. Canvas | +$20.00) BUT this will not work for me because someone ordering a 5x5' canvas print will be paying +$20, whereas someone ordering a giant 20x20' canvas print will only be getting charged the same +$20 too. Is there a way to make it a multiplier? (i.e. Canvas | {weight}x{height}*$20.00)

Thanks a ton fellas... I really appreciate the help.