Abstract
OpenCart is awesome but is not fully prepared to sell by meters, square meters, weights, etc…
For bulk sales comes CFO to help you.

Dummy Case
(It’s only a fragment of a formula)
Take a look at this discount plan:
+1 to 5 m2: 0%
+5 to 10 m2: 10%
+10 to 15 m2: 15%
+15 to 20 m2: 20%
+20 m2: 25%

Now in our formula we will calculate a dummy variable for SquareMeters and then you will find the conditional which handles the discounts per square meters.
Notice that ranges are checked from bigger to smaller.

To know more about conditionals you can read Simple conditionals article.

// It's only a fragment of a formula.
...

// Calculate square meters
$SquareMeters = $cuWidth * $cuHeight;

// Now store price in a variable, so keep the original unchanged
$OurPricePerSquareMeters = $Price; 

// Let's see the discounts
if($SquareMeters>20) {
	// Apply 25%
	$OurPricePerSquareMeters = $OurPricePerSquareMeters - ($OurPricePerSquareMeters*25/100);
} elseif($SquareMeters>15 and $SquareMeters<=20) {
	// Apply 20% 
	$OurPricePerSquareMeters = $OurPricePerSquareMeters - ($OurPricePerSquareMeters*20/100); 
} elseif($SquareMeters>10 and $SquareMeters<=15) {
	// Apply 15% 
	$OurPricePerSquareMeters = $OurPricePerSquareMeters - ($OurPricePerSquareMeters*15/100); 
} elseif($SquareMeters>5 and $SquareMeters<=10) {
	// Apply 10%
	$OurPricePerSquareMeters = $OurPricePerSquareMeters - ($OurPricePerSquareMeters*10/100);
} else {
	// No discount
}

// And calculate final price
$FinalPrice = $SquareMeters * $OurPricePerSquareMeters;