price per square meter, but pay per box

A website I'm working on uses prices per square meter. The products (ceramic tiles) are shipped per box. The boxes contain different percentages of a square meter depending on the product.
I need to display the square meter price. But the customer has to buy a number of boxes, with a price per box showing.
Any ideas?

I'm using the sku property now to display if a product is per square meter or per box.

Hi,

we have developed two extensions in this way:
- PPM (Price per meters & square meters)
- CFO (Custom Formulas)

Please visit our demo store to see they working.
http://esponi.xtrweb.com

Admin zone (demo / demo)
http://esponi.xtrweb.com/admin