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add_filter('woocommerce_is_purchasable', 'ywraq_is_purchasable', 10, 2);
function ywraq_is_purchasable( $is_purchasable, $product ){
	if( !$product){
		return $is_purchasable;
	}

	$price = $product->get_price();
	if( empty( $price ) ){
		return true;
	}

	return $is_purchasable;

}