Magento – Default selected simple product in configurable product

When you create a configurable product it doesn't matter what's the price of simple products -- these prices are ignored completely. So if you want to sell a simple product A which has price $29.99 and a simple product B ($39.99) then you must create a configurable product, set its price to $29.99 and open Associated product tab. Add products you want to associate with this configurable product. After adding them a block named Super product attributes configuration appears which contains options and price differences. Leave product A price empty and put 10 (+$10) to product B price field and voila: different simple product have different price.

There's actually an extension that allows you to use simple product prices instead of price differences, but it's kind of tricky to set up. Since it's free extension, I hope nobody complains about me pasting its link here:

https://github.com/organicinternet/magento-configurable-simple

The price is pulled from catalog/product/price.phtml. The following code will get the lowest price of all associated simple products. This doesn't take in to consideration all of the tax rules in this file though. You could also change the variable $finalPrice in the below code to just be 0 and that would return the smallest amount that would be added on to the configurable.

// Load the full product
$configProduct = Mage::getModel('catalog/product')->load($_product->getId());
// Full product has to be loaded in order to pull configurable attributes
$configAttributes = $configProduct->getTypeInstance(true)->getConfigurableAttributes($configProduct);

// Get the configurable base price
$basePrice = $configProduct->getFinalPrice();
// Create an array to store attribute price values
$attributePrices = array();

// Loop through the configurable attributes
foreach ($configAttributes as $configAttribute) {
    // Grab the pricing information for the configurable attributes
    $simplePrices = $configAttribute->getPrices();
    // Loop through the pricing information for the configurable attributes
    foreach ($simplePrices as $simplePrice) {
        // Check if the additional price is percent or fixed of configurable price
        if ($simplePrice['is_percent']) /* Percent */ {
            // Calculate percentage using base price of configurable product
            $attributePrices[$simplePrice['value_index']] = ((float)$simplePrice['pricing_value'] * $basePrice) / 100;
        } else /* Fixed */ {
            $attributePrices[$simplePrice['value_index']] = (float)$simplePrice['pricing_value'];
        }
    }
}

// Create a array of all simple products associated to the configurable product
$simpleProducts = $configProduct->getTypeInstance()->getUsedProducts();
// Create an array to store simple price final price
$finalPrices = array();

// Loop through all the simple products
foreach ($simpleProducts as $simpleProduct) {
    // Create final price variable and set it to base configurable value
    $finalPrice = $basePrice;
    // Loop through the configurable attributes
    foreach ($configAttributes as $configAttribute) {
        // Get the value of the attribute for the simple product
        $attributeValue = $simpleProduct->getData($configAttribute->getProductAttribute()->getAttributeCode());
        // Get the price of the attribute from the price array
        if (isset($attributePrices[$attributeValue])) {
            // Add the value onto the final price of the simple product
            $finalPrice = $finalPrice + $attributePrices[$attributeValue];
        }
    }

    // Check if simple product is saleable before adding final price to our array
    if ($simpleProduct->isSalable()) {
        // Add the final price to the final prices array
        $finalPrices[$simpleProduct['entity_id']] = $finalPrice;
    }
}

// Gets minimum price of the array 
echo min($finalPrices);