Update product price using a hook in WooCommerce

I am trying to update the product regular price using the meta key _regular_price with an integer or string when the product is updated in the wp-admin.

My desired user-flow is:

1. Open the product edit page
2. Click the update button
3. See that the _regular_price is set to 20 after the page has reloaded.

add_action( 'woocommerce_process_product_meta', 'update_test' );
function update_test( $post_id ) {
    update_post_meta( $post_id, '_regular_price', 20 );
}

Please help me find what I'm doing wrong in the above function and let me know of any other ways to accomplish this.

Updated (August 2018)

Your code is correct but the hook is made for Metaboxes custom fields.

You should use instead save_post_{$post->post_type} Wordpress hook targeting product post type only.

Also You may need to update the active price and to refresh the product transient cache with the function wc_delete_product_transients().

So your code will be:

add_action( 'save_post', 'update_the_product_price', 10, 3 );
function update_the_product_price( $post_id, $post, $update ) {

    if ( $post->post_type != 'product') return; // Only products
    
    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return $post_id;

    // Check the user's permissions.
    if ( ! current_user_can( 'edit_product', $post_id ) )
        return $post_id;

    $price = 50; // <===  <===  <===  <===  <===  <=== Set your price

    $product = wc_get_product( $post_id ); // The WC_Product object

    // if product is not on sale
    if( ! $product->is_on_sale() ){
        update_post_meta( $post_id, '_price', $price ); // Update active price
    }
    update_post_meta( $post_id, '_regular_price', $price ); // Update regular price
    wc_delete_product_transients( $post_id ); // Update product cache
}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

Tested and works…

Addition (2024 update)

You can use woocommerce_admin_process_product_object action hook since WooCommerce 3 as following (which simplify the code):

add_action( 'woocommerce_admin_process_product_object', 'update_the_product_price', 100 );
function update_the_product_price( $product ) {
    $price = 50; // <=== Set your price

    if( ! $product->is_on_sale() ){
        $product->set_price($price); // Update active price
    }
    $product->set_regular_price($price); // Update active price

    // No need to delete product transients, as cache is auto refreshed here.
}