Magento 1.9 – How to Add Different Prices for Different Product Sizes

So what you need is the way to add the simples to the configurable?

// $newids is of form:
$newids = array($id => 1, $id2 => 1);
// $configurableProduct = Mage::getModel('catalog/product);

Mage::getResourceModel('catalog/product_type_configurable')
  ->saveProducts($configurableProduct, array_keys($newids));

Then you can hook into saving of a product. Before save you check wether an ID is given, if not, add a flag. In after_save you check for the flag, if it is set, you load loop over all configurable products and add the new simple one.

To do this you can add tier prices for the product.

Go to the product page in the Magento admin area and navigate to the 'Price' tab. There you will have an attribute called 'Tier Price' where you can configure the price per ordered quantity.

Edit: As per request a solution with shopping cart price rules:

You can do this by making 2 separate rules.

Rule number 1:

Conditions:

Actions:

Rule number 2:

Conditions:

Actions:

Now make sure that rule 2 is executed before rule number 1 by giving it a lower priority and you're done :-) Rule number 1 won't be executed because rule number 2 has 'stop further rules processing' enabled.

You can of course change the conditions to match an attribute like partial SKU or attribute_set instead of a category. This is up to you.