functions.php

add_shortcode( 'price_per_weight', 'wc_custom_per_price_weight' );
function wc_custom_per_price_weight() {
	global $product;

	if ( $product ) {
		$price  = $product->get_price();
		$weight = $product->get_weight();

		$price_kg = $price / $weight;

		return wc_price( $price_kg );
	}
}