Set a Default Price for Products

functions.php

<?php
/**
 * Sets the product default price to 10. Only works once, if the price is not specified.
 *
 * @param int    $post_id
 * @param object $post
 */
function set_product_default_price( $post_id, $post ) {
	$product     = wc_get_product( $post_id );
	$already_set = get_post_meta( $post_id, '_set_default_price', true );
	$price       = $product->get_price();

	if ( 'yes' !== $already_set && empty( $price ) ) {
		$product->set_regular_price( '10' );
		$product->save();

		update_post_meta( $post_id, '_set_default_price', 'yes' );
	}
}
add_action( 'woocommerce_process_product_meta', 'set_product_default_price', 999, 2 );