Get the lowest and highest possible price for a configurable product

Try this approach. use the config array that the attribute dropdowns use to change the price of the configurable product. Let's assume that $productId is the ID of the configurable product.

$product = Mage::getModel('catalog/product')->load($productId);
$block = Mage::app()->getLayout()->createBlock('catalog/product_view_type_configurable');
$block->setProduct($product);
$config = json_decode($block->getJsonConfig(), true);
$basePrice = $config['basePrice'];
$min = null;
$max = null;
foreach ($config['attributes'] as $aId=>$aValues){
    foreach ($aValues['options'] as $key=>$value){
        if (is_null($min) || $min>$value['price']){
            $min = $value['price'];
        }
        if (is_null($max) || $max<$value['price']){
            $max = $value['price'];
        }
    }
}
//until here we have the min and max price differences. Now just add the base price.
$min += $basePrice;
$max += $basePrice;

A simpler one.

if($_product->isConfigurable()) // check if the product is configurable (u may check this in for loop of products)
                {
                    //get associative (child) products
                    $childProducts = Mage::getModel('catalog/product_type_configurable')->getUsedProducts(null,$_product);
                    $childPriceLowest = "";    
                    $childPriceHighest = "";       
                    foreach($childProducts as $child){
                        $_child = Mage::getModel('catalog/product')->load($child->getId());

                        if($childPriceLowest == '' || $childPriceLowest > $_child->getPrice() )
                        $childPriceLowest =  $_child->getPrice();

                        if($childPriceHighest == '' || $childPriceHighest < $_child->getPrice() )
                        $childPriceHighest =  $_child->getPrice();

                    }
                    $price_array = array($childPriceLowest,$childPriceHighest); // array containing required values
                }

This answer, also by @Marius, to a similar question is a good basis to work from. Using that, here's a solution to this problem that takes into account the potential for configurable products having multiple attributes that change price.

I've written it as function that takes a configurable product id, and returns a string of min to max price. It should be pretty clear how to work it into the context that you need.

function getPriceRange($productId) {

 $max = '';
 $min = '';

 $pricesByAttributeValues = array();

 $product = Mage::getModel('catalog/product')->load($productId); 
 $attributes = $product->getTypeInstance(true)->getConfigurableAttributes($product);
 $basePrice = $product->getFinalPrice();

 foreach ($attributes as $attribute){
    $prices = $attribute->getPrices();
    foreach ($prices as $price){
        if ($price['is_percent']){ //if the price is specified in percents
            $pricesByAttributeValues[$price['value_index']] = (float)$price['pricing_value'] * $basePrice / 100;
        }
        else { //if the price is absolute value
            $pricesByAttributeValues[$price['value_index']] = (float)$price['pricing_value'];
        }
    }
 }


 $simple = $product->getTypeInstance()->getUsedProducts();

 foreach ($simple as $sProduct){
    $totalPrice = $basePrice;

    foreach ($attributes as $attribute){

        $value = $sProduct->getData($attribute->getProductAttribute()->getAttributeCode());
        if (isset($pricesByAttributeValues[$value])){
            $totalPrice += $pricesByAttributeValues[$value];
        }
    }
    if(!$max || $totalPrice > $max)
        $max = $totalPrice;
    if(!$min || $totalPrice < $min)
        $min = $totalPrice;
 }

 return "$min - $max";

}