Mostrar Precio de Oferta Antes del Precio Regular (WooCommerce)

Debes cambiar esto en functions.php

if (!function_exists('my_commonPriceHtml')) {

function my_commonPriceHtml($price_amt, $regular_price, $sale_price) {
    $html_price = '<p class="price">';
    //si el producto está en oferta
    if (($price_amt == $sale_price) && ($sale_price != 0)) {
        $html_price .= '<ins>' . wc_price($sale_price) . '</ins>';
        $html_price .= '&nbsp;&nbsp;<del>' . wc_price($regular_price) . '</del>';
    }
    //en oferta pero gratis
    else if (($price_amt == $sale_price) && ($sale_price == 0)) {
        $html_price .= '<ins>¡Gratis!</ins>';
        $html_price .= '<del>' . wc_price($regular_price) . '</del>';
    }
    //no está en oferta
    else if (($price_amt == $regular_price) && ($regular_price != 0)) {
        $html_price .= '<ins>' . wc_price($regular_price) . '</ins>';
    }
    //para producto gratuito
    else if (($price_amt == $regular_price) && ($regular_price == 0)) {
        $html_price .= '<ins>¡Gratis!</ins>';
    }
    $html_price .= '</p>';
    return $html_price;
}

}

add_filter('woocommerce_get_price_html', 'my_simple_product_price_html', 100, 2);

function my_simple_product_price_html($price, $product) {
if ($product->is_type('simple')) {
    $regular_price = $product->regular_price;
    $sale_price = $product->sale_price;
    $price_amt = $product->price;
    return my_commonPriceHtml($price_amt, $regular_price, $sale_price);
} else {
    return $price;
}
}

add_filter('woocommerce_variation_sale_price_html', 'my_variable_product_price_html', 10, 2);
add_filter('woocommerce_variation_price_html', 'my_variable_product_price_html', 10, 2);

function my_variable_product_price_html($price, $variation) {
$variation_id = $variation->variation_id;
//creando el objeto producto
$variable_product = new WC_Product($variation_id);

$regular_price = $variable_product->regular_price;
$sale_price = $variable_product->sale_price;
$price_amt = $variable_product->price;

 return my_commonPriceHtml($price_amt, $regular_price, $sale_price);
 }

add_filter('woocommerce_variable_sale_price_html', 'my_variable_product_minmax_price_html', 10, 2);
add_filter('woocommerce_variable_price_html', 'my_variable_product_minmax_price_html', 10, 2);

function my_variable_product_minmax_price_html($price, $product) {
$variation_min_price = $product->get_variation_price('min', true);
$variation_max_price = $product->get_variation_price('max', true);
$variation_min_regular_price = $product->get_variation_regular_price('min', true);
$variation_max_regular_price = $product->get_variation_regular_price('max', true);

if (($variation_min_price == $variation_min_regular_price) && ($variation_max_price == $variation_max_regular_price)) {
    $html_min_max_price = $price;
} else {
    $html_price = '<p class="price">';
    $html_price .= '<ins>' . wc_price($variation_min_price) . '-' . wc_price($variation_max_price) . '</ins>';
    $html_price .= '<del>' . wc_price($variation_min_regular_price) . '-' . wc_price($variation_max_regular_price) . '</del>';
    $html_min_max_price = $html_price;
}

return $html_min_max_price;
}

Lo he intentado yo mismo y este es mi código price.php. Solo necesitas agregar algo de CSS según tus necesidades.

Primero, elimina el código existente

'<p class="'.esc_attr( apply_filters( 'woocommerce_product_price_class', 'price' ) ).'">'.$product->get_price_html().'</p>';

Luego coloca este código:

global $product;
$product_id = $product->get_id();

if( $product->is_type( 'simple' ) ){
    $regular_price = $product->get_regular_price();
    $sale_price = $product->get_sale_price();
    $discount = ($sale_price/$regular_price)*100;

echo    '<ul class="'.esc_attr( apply_filters( 'woocommerce_product_price_class', 'price')).'">
            <li class="sale_price"><span class="woocommerce-Price-currencySymbol">'.get_woocommerce_currency_symbol().'</span>'.$sale_price.'</li>
            <li class="regular_price"><span class="woocommerce-Price-currencySymbol">'.get_woocommerce_currency_symbol().'</span>'.$regular_price.'</li>
            <li class="disc">'.round($discount,0).'% Descuento</li>
            <div class="clear"></div>
        </ul>';
} elseif( $product->is_type( 'variable' ) ){
    // Precio regular mínimo y máximo
    $min_regular_price = $product->get_variation_regular_price( 'min' );
    $max_regular_price = $product->get_variation_regular_price( 'max' );

    // Precio de oferta mínimo y máximo
    $min_sale_price = $product->get_variation_sale_price( 'min' );
    $max_sale_price = $product->get_variation_sale_price( 'max' );

    // El precio activo mínimo y máximo
    $min_price = $product->get_variation_price( 'min' );
    $max_price = $product->get_variation_price( 'max' );
    $var_discount = ($min_sale_price/$min_regular_price)*100;

echo    '<ul class="'.esc_attr( apply_filters( 'woocommerce_product_price_class', 'price')).'">
            <li class="sale_price"><span class="woocommerce-Price-currencySymbol">'.get_woocommerce_currency_symbol().'</span>'.$min_sale_price.'</li>
            <li class="regular_price"><span class="woocommerce-Price-currencySymbol">'.get_woocommerce_currency_symbol().'</span>'.$max_regular_price.'</li>
            <li class="disc">'.round($var_discount,0).'% Descuento</li>
            <div class="clear"></div>
        </ul>';
    }

El resultado es:

El precio proviene del archivo price.php en el plugin de WooCommerce. Dentro de este archivo hay una función como:

$price_html = $product->get_price_html();

Debes comentarla y colocar el siguiente código en lugar de esta función.

// Precio de oferta
<span class="price"><?php echo "Precio de oferta:". " " . get_woocommerce_currency_symbol().$product->get_sale_price(); ?></span>
// Precio normal
<span class="price"><?php echo "Precio normal:". " " . get_woocommerce_currency_symbol().$product->get_regular_price(); ?></span> 

y añade las clases según tus necesidades.