add_filter('woocommerce_get_price_html', 'changeFreePriceNotice', 10, 2);

function changeFreePriceNotice($price, $product) {
	if ( $price == wc_price( 0.00 ) )
		return '<span style="color: #4b9db3; font-size: 20px;"><strong>Цена по запросу</strong></span>';
	else
		return $price;
}